Boil butter, milk and 100 ml of cream in a saucepan, stir in the polenta and let it swell over low heat.
Season chicken breasts with salt and pepper well. Fry the chicken breasts over medium heat on each side for about 6 minutes. Wrap them in aluminum foil and let it rest.
Peel onion and garlic and chop finely. Clean mushrooms and quarters , depending on their size. Roast the onion, garlic and mushrooms until golden brown in the frying residue. Stir in the remaining cream and simmer until a creamy sauce is produced. Season to taste.
Serve and garnish with sage and thyme.