Combine tomato sauce, brown sugar, vinegar and mustard in a small bowl; set aside. Combine egg, onion, cracker crumbs, ground beef, salt, and pepper in a mixing bowl. Add 1/2 of tomato sauce mixture. Blend thoroughly. Shape into a loaf. Place in microwave safe bowl. Make depression in top of loaf. Pour remaining tomato sauce mixture on top of loaf. Cook uncovered for 30 mintues. Take out of microwave and cover. Let stand for 10 minutes before slicing.