Chop the onions, press the garlic and sauté both with oil in a pan. Add the oregano. Chop the tomatoes and mushrooms and add them. Braise the vegetables for about 2 minutes over high heat, season with salt and pepper and place in a baking dish.
Drizzle the fish fillets with lemon juice and season with salt and pepper. Place the fish on the vegetables and brush with the sour cream.
Cover and cook in a preheated oven at 200 ° C top / bottom heat for 20 minutes.