Mix sauce ingredients, set aside until meatloaf is halfway cooked.
Beat eggs; stir in milk, oats, sage, salt, onion and optional celery. Mix into ground beef. NOTE: Traditional meatloaf has double the salt; we like this recipe as is, but you may season to your own tastes by increasing salt or adding pepper.
Place in 9" X 9" pan, leaving 1/2" empty border (makes it easy to remove any grease later).
Bake uncovered for 30 minutes at 350 degrees. Cover with sauce and bake an additional 30 minutes (still uncovered) at 350 degrees.
Immediately remove any grease standing in the border.