I use the same mixture for fried hamburger patties and/or meatloaf.
For each pound of ground beef (usually 80/20)
(all measurements are approximate)
1/2 cup chopped onion - use cooking onions, if available, not sweet onions
1 T canned chopped garlic
1 egg
1 tsp salt
1 tsp sugar (it enhances the other flavors)
1/4 tsp ground black pepper
1/2 tsp (or so) ground sage
1/4 cup milk
1/4 cup uncooked oatmeal
I usually use three pounds of ground beef when mixing this up.
This ends up being quite firm - so it slices well for sandwiches the next day.