Keto Sea Trout in Foil with Fennel and Herb Butter
Cooking in aluminum foil (en papillote) is the most reliable method for tender fish: it steams in its own moisture and never dries out. Sea trout has a similar omega-3 profile to salmon but a milder flavor. Fennel softens and lightly caramelizes as it cooks - just 4 g net carbs for the whole dish.
Key facts at a glance
- Prep and cook time: approx. 30 minutes - 10 minutes prep, 20 minutes in the oven.
- Approx. 4 g net carbs and 50 g fat per serving.
- Ingredients: sea trout fillet, fennel, butter, dill, lemon, garlic, olive oil.
- Foil cooking makes the fish foolproof - it's almost impossible to overcook.
Ingredients
One serving equals one foil packet containing the whole sea trout fillet and fennel - a complete keto dinner.
| Amount | Ingredient | Note | ||
|---|---|---|---|---|
| Sea trout fillet (boneless, skin on) | - | |||
| Fennel (thinly sliced) | - | |||
| Butter (softened) | - | |||
| Olive oil | approx. 1 tbsp | |||
| approx. 1 tbsp | ||||
| Lemon (sliced) | - | |||
| Fresh dill (or 1 tsp dried) | 2 sprigs | |||
| 2 sprigs | ||||
| Garlic clove | finely chopped | |||
| finely chopped | ||||
| Salt | to taste | |||
| to taste | ||||
| Pepper | to taste | |||
| to taste | ||||
Preparation
-
Preheat the oven and mix the herb butter
approx. 5 minutes25 g Butter (softened) 15 ml Olive oil approx. 1 tbsp 10 g Fresh dill (or 1 tsp dried) 2 sprigs 1 clove Garlic clove finely chopped 0.3 tsp Pepper to taste Preheat the oven to 200 °C (400 °F) on top/bottom heat. Stir softened butter (25 g) with finely chopped dill (or 1 tsp dried), garlic, and pepper into an herb butter. Spread aluminum foil (40 x 50 cm, doubled) on a baking sheet and drizzle olive oil (1 tbsp) in the center.
-
Build the foil packet
approx. 5 minutes260 g Sea trout fillet (boneless, skin on) 150 g Fennel (thinly sliced) 0.5 piece Lemon (sliced) 0.5 tsp Salt to taste Lay the fennel slices (150 g) on the olive oil and place the sea trout fillet (260 g, skin side down) on top. Season with salt and pepper. Spread the herb butter over the fish and lay the lemon slices on top. Fold the foil tightly around the fish and crimp every edge firmly several times - no steam should escape.
-
Bake and plate
approx. 20 minutesSlide the foil packet into the preheated oven and bake for 18-20 minutes. The foil will puff up slightly from the steam - that's exactly what you want. Take it out and let it rest for 2 minutes. Open the packet carefully at the table - the steam will escape. Transfer the fish and fennel to the plate and drizzle the juices from the foil over the top as a sauce.
You did it! Enjoy every delicious bite.
Which tips help with keto sea trout in foil?
- Cut a generous piece of aluminum foil (approx. 40 x 50 cm) and use a double layer - a tight packet keeps the steam in.
- Place the fennel under the fish so it acts as a bed and the fish doesn't sit directly on the foil - that prevents sticking.
- Leave the herb butter on top of the fish, don't smear it on the foil - so the butter melts down through the flesh as it cooks.
- Bake at 200 °C (400 °F) on conventional top/bottom heat (not convection) - convection dries fish out in foil.
- Open the foil packet only 2 minutes before serving - that keeps the fish hot all the way to the table.
Why sea trout in foil fits keto so well
A 260 g sea trout fillet provides about 51 g protein and 8 g fat - sea trout (rainbow trout) has slightly less fat than salmon but similar omega-3 levels. 150 g of fennel contains about 3 g net carbs and supplies potassium and fiber, plus its signature anise aroma. Herb butter (25 g butter + dill + garlic) adds about 21 g fat. 1 tbsp of olive oil in the foil packet brings another 14 g of fat. Lemon juice contributes less than 1 g of carbs. Total: approx. 52 g protein, 50 g fat, 4 g net carbs, around 660 kcal.
Source: USDA FoodData Central - Trout, rainbow, farmed, cooked , FDDB Nutrition Database - Fennel
Sea trout: omega-3 source and keto protein
Rainbow trout (sea trout) contains around 0.7-1.2 g of omega-3 fatty acids (EPA + DHA) per 100 g - comparable to salmon but usually less expensive. According to the NIH, omega-3 fatty acids support heart health and may reduce inflammatory markers. On a keto diet, fatty fish is a key source of long-chain omega-3s, which the body can only synthesize in limited amounts from plant sources.
Source: NIH Office of Dietary Supplements - Omega-3 Fatty Acids
Fennel as a keto vegetable: anise-scented and low in calories
Fennel has about 3 g of net carbs per 100 g (7 g total carbs minus roughly 4 g of fiber) and contains anethole, which gives it that characteristic anise aroma. Cooked in foil, fennel turns soft and slightly sweet. Not a fan of anise? Swap the fennel for zucchini (even fewer carbs).
How do I serve and vary the sea trout from foil?
Cut open the foil packet right on the plate - the resulting blend of fish juices, butter, and lemon makes a natural sauce. From my own experience, I recommend opening the packet at the table: the burst of aroma is part of the pleasure.
Meal prep: assembling the foil packets ahead
The foil packets can be assembled up to 4 hours before baking and stored in the fridge - handy for weeknight dinners. Slide them into the oven only when you're ready to serve. Leftovers keep for 1 day in the fridge and can be eaten cold (as a fish salad) or gently reheated.
Salmon or sea bream variation
Salmon fillet (260 g) is the richest substitute - more omega-3, a stronger flavor, slightly higher calories. Sea bream fillet (260 g) is leaner with delicate white flesh and a mild ocean flavor. Cod fillet is the leanest option with more protein and less fat - increase the herb butter to 35 g.
Nutrition values
| Nutrient | Per 100 g | Per serving | Keto context |
|---|---|---|---|
| Calories | approx. 147 kcal | approx. 660 kcal | Complete dinner; energy from protein and fat (fish, butter, olive oil). |
| Complete dinner; energy from protein and fat (fish, butter, olive oil). | |||
| Fat | approx. 11 g | approx. 50 g | Omega-3 fatty acids from sea trout; saturated fat from butter. |
| Omega-3 fatty acids from sea trout; saturated fat from butter. | |||
| Net carbs | approx. 0.9 g | approx. 4 g | Very low; almost entirely from fennel. |
| Very low; almost entirely from fennel. | |||
| Sugar | approx. 0.5 g | approx. 2 g | Natural sugar from fennel. |
| Natural sugar from fennel. | |||
| Protein | approx. 11.5 g | approx. 52 g | Very high; sea trout fillet is the main protein source. |
| Very high; sea trout fillet is the main protein source. | |||
| Salt | approx. 0.3 g | approx. 1.3 g | Low; only from seasoning - fish needs little salt. |
| Low; only from seasoning - fish needs little salt. | |||
Note: Nutrition values are estimated averages per 100 g and per serving and may vary depending on ingredients, brands, portion size, and preparation. They do not replace individual nutrition or medical advice.
FAQ
What's the difference between sea trout and salmon?
Sea trout (rainbow trout) is a freshwater fish, while salmon is a saltwater fish. Sea trout has a similar salmon-orange flesh thanks to carotenoid-rich feed. It contains a bit less fat than Atlantic salmon but comparable omega-3 levels, and it's often cheaper. The flavor is milder and more delicate than salmon.
How do I tell when the fish in the foil packet is done?
Gently test the fish in the middle with a fork: the flesh should flake into layers and look opaque white-pink. A 260 g fillet (2-3 cm thick) needs 18-20 minutes at 200 °C (400 °F). Opening the packet too early lets the steam escape - it's better to cook it a little longer.
Can I swap the herb butter for olive oil?
Yes, 3 tbsp of olive oil instead of herb butter is a dairy-free alternative. The flavor is slightly different (less creamy), but the fish quality and doneness stay the same. Always add the dill and garlic - they're essential to the dish's character.
Is fennel a good keto side for fish?
Yes, fennel (3 g net carbs per 100 g) is one of the most aromatic keto vegetable sides for fish dishes. Its anise note works especially well with delicate fish like trout, sea bream, and sea bass. If you don't like anise, swap in zucchini or spinach - both have even fewer carbs.