Clean the mushrooms and spring onions and cut into slices or rings. Mix both with the cream cheese and egg yolk. Season with salt, pepper and lemon juice. Clean the fish and also season with salt, pepper and lemon juice.
Place the fillets side by side in a lightly buttered casserole dish and brush with the cheese cream. Bake in a preheated oven at 200 degrees for about 20 minutes. This goes well with salad and baguette.