Poach bone-in chicken breasts with diced white onion and celery seasoned with salt and pepper. Remove skin and debone, place meat along with vegetables and one or two cloves of roast garlic in processor. Mix thoroughly, fluff with a fork and mix well with ricotta cheese and sour cream. Moisten with velouté sauce or stock and season with white pepper and thyme. Use mixture to fill cooked manicotti. Place stuffed pasta in a baking dish, top with Mornay. Bake until bubbling and browned.