3⁄4 l water and approx. 1⁄2 Bring tsp. salt to the boil. Stir in stock. Add polenta while stirring, bring to the boil and simmer at low heat for about 10 minutes. Stir frequently. Take polenta off the stove and let it swell for about 10 minutes.
Wash the parsley and rosemary and shake dry. Pluck off leaves or needles and chop. Stir the herbs into the polenta. Season to taste with salt, pepper and nutmeg. Line a baking tray with baking paper. Spread the polenta about 1 cm thick on top to form a large pizza base and let it cool down.
In the meantime clean the mushrooms, possibly wash and halve them. Heat butter in a large pan, fry mushrooms in it in 2 portions while turning. Season with salt and pepper.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Peel onions and cut into thin rings. Spread polenta with sour cream. Spread onions and mushrooms on top. Cover with cheese. Bake in a hot oven for 15-20 minutes.