Clean the mushrooms, wash if necessary and cut into slices. Peel and finely chop the onions. Peel and chop garlic. Dab schnitzel dry.
Heat the oil in portions in a large frying pan. Fry the escalopes on each side. Season with salt and pepper and place them side by side in a large greased dish (approx. 38 cm long).
Fry the mushrooms in hot frying fat in portions. Sauté onions, garlic and dried marjoram. Season with salt and pepper and remove.
Heat 60 g butter in the frying fat. Sweat flour in it. Stir in 1 1/2 l water and stock. Bring to the boil and simmer for about 5 minutes. Stir in sour cream. Add mushrooms again, season to taste. Fold in half breadcrumbs.
Spread the mushroom sauce evenly over the escalopes. Bake in the pre-heated oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2) covered for about 1 hour. Then cook open for about 1 hour more. About 30 minutes before the end of the cooking time, spread 50 g butter in flakes and the rest of the breadcrumbs on top and finish cooking.
(If the crust gets too dark, cover again.) Arrange the schnitzel and garnish with marjoram. Fried potatoes taste good with it.
Drink tip: cool white wine, e.g. a Pinot Blanc