1. Preheat the oven to 180°C (350°F).
2. Steam the peeled sweet potatoes (10 minutes in the microwave + 2 minutes rest), mash them and add milk, nutmeg, salt, and pepper.
3. Place the white fish fillets in a microwave-safe dish (mine were frozen), sprinkle them with fish stock and 1 teaspoon of olive oil. Cook covered for 4 minutes, then let rest for 2 minutes.
4. Chop the shallot, place it in a microwave-safe dish and mix with 1 teaspoon of olive oil, cook covered in the microwave for 2 minutes and let rest for 2 minutes.
5. Mash the fish with a fork.
6. Mix the mushrooms with the shallot, garlic, salt, and pepper. Cook for another 2 minutes.
7. Spread half of the sweet potato mash in the bottom of a gratin dish, add the fish, mushrooms, and cover with the remaining mash. Sprinkle with Parmesan cheese.
8. Bake in the oven for 30 minutes.
9. Serve with a green salad.