Set oven to 350 degrees F.
Whisk eggs and milk together in a bowl.
Add chopped onion, shredded cheese, and ground beef to the egg and milk mixture.
Form the mixture into a loaf, and place on a baking sheet or in a large baking dish.
Bake for 50 minutes.
While the meatloaf is baking, work on the topping. In a small bowl, mix yellow mustard, ketchup, and brown sugar.
After about 30 minutes into baking, open the oven and spread half the mixture onto the meatloaf. After 10 minutes, add the rest of the mixture to the top of the meatloaf, and finish baking.