Wash mushrooms, dry. Finely chop the mushroom legs. Finely chop the boiled chicken. Peel the onion and chop finely. Cheese grate on a fine grater.
Heat the vegetable oil, fry the onion and chopped champignon legs, add the chopped chicken breast and cook for another 4-5 minutes.
Stuff the mushroom caps with the filling, sprinkle generously with cheese on top. Put the mushrooms in a baking dish and cook in an oven preheated to 200 degrees for 25-30 minutes.