Wash the spinach. Sauté the onion and garlic finely diced in the oil, add the washed spinach and let it collapse.
Butter a baking dish and put a slice of fish in it. Drizzle with lemon juice, season with salt. Cover these with spinach. Place the second slice of fish on top and season with lemon juice and salt.
Mix the créme fraîche and parmesan, carefully season with salt. Spread this topping on the fish and bake in the preheated oven at 200 ° for 35 - 40 minutes.