Heat oil in a skillet. Cook onion and pepper 6 minutes until soft; cool 10 minutes.

Mix beef, cooked vegetables, breadcrumbs, eggs, milk, and salt with fingertips just until combined.

Shape into a loose loaf on a rimmed sheet pan. Bake at 350°F for 35 minutes.

Mix ketchup, brown sugar, and vinegar; brush half over loaf. Bake 15 minutes.

Brush with remaining glaze; bake 10 to 15 minutes until center reaches 160°F. Rest 15 minutes before slicing.