Heat the oil in a skillet and cook the onion and bell pepper for 6 minutes until soft, then cool them for 10 minutes.
Mix the beef, cooked vegetables, breadcrumbs, eggs, milk and salt with your fingertips just until combined.
Shape the mixture into a loose loaf on a rimmed sheet pan and bake at 350°F for 35 minutes.
Stir the ketchup, brown sugar and vinegar, brush half over the loaf, and bake for 15 minutes.
Brush on the remaining glaze and bake for 10 to 15 minutes until the center reaches 160°F, then rest for 15 minutes before slicing.