Preheat your oven to 375°F (190°C).
In a skillet over medium heat, add olive oil. Sauté onions, bell pepper, and garlic until they become translucent.
In a large mixing bowl, combine ground beef, breadcrumbs, milk, eggs, Worcestershire sauce, salt, and pepper. Mix well.
Add in the sautéed onion mixture and blend until just combined.
Mold the meat mixture into the loaf pan.
For the glaze, in a separate bowl, mix together ketchup, brown sugar, vinegar, and mustard. Spread half of this mixture over the meatloaf.
Bake for about 1 hour, or until the meatloaf is cooked through and reaches an internal temperature of 160°F (71°C).
Halfway through baking, spread the remaining glaze over the meatloaf.
Once done, remove it from the oven and allow it to rest for 10 minutes before serving.