Preheat oven to 350F. Slice the french bread in half lengthwise. Press down the inside of each half with your fingers, (or remove some of the bread) creating an indention to hold the meat mixture. Place each half onto a baking sheet.
Brown the ground beef with the onion until the beef is done, remove from heat, and drain off the fat. Mix in the evaporated milk, egg, oats, mustard, steak sauce, salt, pepper, and 2 cups cheddar cheese.
Stuff equally into each half, top with remaining 1 cup cheese, and bake for about 20 minutes. Midway through cooking, check to see if the edges of the bread are browning too quickly. If so, loosely cover the loaves with foil and continue baking.
After baking, slice into hand-held size serving pieces.