Add the carrots, celery, onion and garlic to the casserole and cook over moderate heat, stirring occasionally, until the vegetables have softened, about 8 minutes. Add the brandy and cook until evaporated, 1 to 2 minutes. Add the remaining 6 cups of stock and the thyme and bay leaf and season with salt and pepper. Return the chicken and bacon to the casserole along with the 2 cups of deglazed drippings and bring to a simmer. Cover partially and cook over moderate heat for 30 minutes. Whisk in the bacon fat and flour paste and cook until the sauce has thickened, 5 to 6 minutes.