In a large mixing bowl, combine meat, bread crumbs, onion, ketchup, eggs, and onion soup mix.
Use your hands to squish all of the ingredients together until all of the ingredients are well combined.
Form the meat mixture into two "loaves." At this point, I usually wrap one of the loaves in press and seal freezer wrap and toss it in the freezer. However, if you have a larger family, you'll probably want to cook both.
Put the two loaves in a 13x9 baking pan and bake for 45 minutes.
While the loaves are baking, mix up the sauce by stirring the ketchup, the brown sugar, and the mustard together until smooth. Set aside.
After 45 minutes, take the meat loaves out and spread the sauce over the tops.
Placed the sauced loaves back in the oven and bake for another 15 minutes or until cooked through.
Let the meat loaves cool for 5 - 10 minutes before cutting them.
P.S. To cook a frozen meatloaf, take the frozen meat loaf out the day before you want to cook it and let it thaw in the refrigerator. Cook as stated above.
P.S.S. This meatloaf pairs wonderfully with my super yummy corn casserole!