Poach chicken breasts with diced white onion and celery seasoned with salt and pepper. When cooked, remove skin and debone, put meat along with vegetables and one or two cloves of roast garlic in processor until thoroughly mixed, fluff with a fork and mix well with ricotta cheese and sour cream. Moisten with a velouté sauce made with chicken stock; season with white pepper and thyme. Use this mixture to fill cooked manicotti. Place stuffed pasta in a baking dish, top with a Mornay made with a good dry cheese and bake in a hot oven until bubbling and browned.