Cook onion, celery and carrots in a few teaspoons of oil in a large nonstick skillet over moderate heat until softened.
Transfer vegetables to a large bowl and stir in breadcrumbs, parsley, egg, ketchup, steak sauce, 1 teaspoon salt and pepper.
Stir in buffalo (do not over mix) and form mixture into a 10 inch x 4 inch oval loaf and place in a shallow metal baking pan.
Bake at 375°F for 1 hour 10 minutes, or until a thermometer inserted 2 inches into the center registers 160°F.
While meatloaf is resting, make sauce by de-glazing the baking pan with water over moderate heat, scraping up brown bits. Once liquid has reduced and slightly thickened, drizzle over meatloaf and serve.