In large bowl mix, egg, sauce, milk, onion, pepper, celery,bread crumbs, salt and pepper together. Crumble ground beef into this. Shape this into 2 (two) loaves. Place these into a 9x13 inch pan. Or can freeze one at this point.
Bake at 350 degrees for 1-1/2 hours. Remove from oven during last ten minutes of baking. Whisk ketchup, mustard, and brown sugar together, Pour over meatloaf, and return to oven for remaining 10 minutes.
Allow to cool slightly and will slice better. Great for cold meatloaf sandwiches.