Preheat the oven to 350 degrees. Line a baking sheet or casserole dish with aluminum foil or parchment paper.
Place the ground beef, onions, and breadcrumbs in a large mixing bowl. Pour the milk and eggs directly over the breadcrumbs, allowing them to soak in the moisture. Sprinkle with salt and pepper. Use your hands to gently combine and bring the mixture together.
Form the mixture into a loaf shape on the prepared pan, about 9-10 inches long, and 4-5 inches wide.
In another bowl, whisk together ketchup, brown sugar, and mustard. Slather the mixture all over the top of the meatloaf.
Bake for 55-70 minutes or until the glaze is caramelized. (A wider, shallower loaf will take less time to bake, and a thicker loaf will take more time). The meatloaf is done when a meat thermometer inserted into the thickest part of the meatloaf reaches 160 degrees. Allow to rest for 10 minutes before slicing.