Preheat oven to 325°F.

In a large bowl, combine the ground beef, onion mixture, bread crumbs, and eggs and mix lightly with a fork. Don't mash, or the meatloaf will be dense.

Shape the mixture into a rectangular loaf on a sheet pan lined with parchment paper. Spread the ketchup evenly on the top and bake for 1 - 1 1/4 hours, until the central temperature is 160°F.

(Cooking with a bowl of water under the pan will stop the top from cracking.)