Preheat the oven to 350 degrees F (175 degrees C). Lightly coat a 9x5-inch loaf pan with cooking spray.

Combine onion, bell pepper, milk, eggs, garlic, 2 tablespoons ketchup, parsley, 3 teaspoons Worcestershire sauce, thyme, and Dijon in a large bowl and mix well. Stir in bread crumbs. Add ground sirloin, salt, and pepper; mix until just combined. Form mixture into a loaf shape and fit into the prepared pan.

Combine remaining ketchup with balsamic vinegar and remaining 1 teaspoon Worcestershire in a small bowl. Mix until well combined and spread over the top of the meatloaf.

Bake in the preheated oven until an instant read thermometer inserted in the center reads 160 degrees F (71 degrees C), about 1 hour. Let stand for 10 minutes before slicing and serving. The longer it sits, the easier it will be to slice.