Cook onion, celery, carrot, and garlic in 2 teaspoons oil in a large nonstick skillet over moderate heat until onion is softened. Transfer to a large bowl and stir in bread crumbs, parsley, egg, ketchup, Worcestershire sauce, salt, and pepper. Stir in buffalo (do not overmix) and form mixture into a 10- by 4-inch oval loaf in a large shallow metal baking pan. Toss shallots and tomatoes with remaining teaspoon oil and salt and pepper to taste, then scatter around meat loaf. Bake in middle of oven 1 hour and 10 minutes, or until a thermometer inserted 2 inches into center registers 160°F. Transfer meat loaf and vegetables to a platter and let stand 10 minutes.
While meat loaf is standing, deglaze baking pan with water over moderate heat, scraping up brown bits, and pour through a fine sieve into a bowl.
Serve meat loaf with shallots, tomatoes, and sauce.
Each serving about 221 calories and 8 grams fat Nutritional analysis provided by Self