Heat the oven to 350°F and line a rimmed sheet pan with foil.
Mix the beef, cracker crumbs, onion, bell pepper, eggs, milk, Worcestershire, salt, pepper and 1/4 cup ketchup with your hands just until evenly combined.
Shape the mixture into a 9-by-5-inch loaf on the pan without packing it hard; smooth the top so thin edges do not burn.
Whisk the remaining 1/2 cup ketchup with the brown sugar and mustard and brush one-third of it over the loaf.
Bake for 35 minutes, brush on another third of the glaze and bake for 15 minutes more.
Brush on the remaining glaze and continue baking until the center reaches 160°F, 10 to 15 minutes.
Rest the meatloaf for 15 minutes before slicing so the juices stay in the meat.