For the polenta, boil 400 ml water, milk and 1 teaspoon salt in a pot. Stir in the polenta and bring to the boil. Let it swell for 10-15 minutes at low heat, stirring from time to time.
Coat a baking tray with oil. Stir the ricotta into the polenta, season with salt. Spread the hot mixture on the baking tray to form a rectangle (approx. 30 x 32 cm), cover and leave to cool for approx. 1 hour.
Onions peel and chop finely. Clean and chop the mushrooms. Wash parsley, shake dry and chop finely.
Cut polenta into 16 rectangles. Heat butter bit by bit in a large pan. Fry the polenta pieces in it in portions on each side for 2-3 minutes until golden brown.
In the meantime heat 2 tablespoons of oil in a deep frying pan. Fry the mushrooms for about 5 minutes while turning them. Fry the onions briefly. Season with salt and pepper. Dust with flour and sauté briefly.
Add 300 ml water while stirring and bring to the boil. Stir in sour cream. Bring to the boil and simmer for about 5 minutes. Stir in mustard, season to taste with salt, pepper and 1 pinch of sugar. Stir in parsley.
Arrange the mushrooms with the ricotta polenta.