Boil the chicken in a large pot, then remove and cool it. Remove the skin and bones and cut the meat into pieces. Season it with salt and pepper. Fry the onions in oil until golden. Add the garlic and braise it for another minute.
Sprinkle it with salt and pepper and add the flour, stir and then top it up with a little milk, while gradually stirring. In a tray, mix the chicken, onions, zucchini, mushrooms and pour on the sauce.
Sprinkle it with cheese and bake in a preheated 392°F (200 °C) oven for 40 minutes. Serve the chicken with warm.