Clean the mushrooms and cut them in slices. Cut the spring onions into rings, set aside 2 tablespoons of the green rings, sauté the remaining rings briefly in the hot oil. Add the mushrooms and cook for 2 - 3 minutes. Dust with flour, add spinach and cream. Let it warm up briefly, season well and place in a baking dish.
Rinse the fish, pat dry. Drizzle lemon juice over the fish and season with salt and pepper. Place the fish on top of the vegetables and cook in the oven for 20 minutes. Sprinkle with the green onion rings.