Meatloaf al Italiano

Name: Meatloaf al Italiano
Ingredients: 1 tablespoon olive oil, or as needed; 1 cup chopped mushrooms; 0.5 cup chopped red bell pepper; 0.5 cup finely chopped onion; 1 clove garlic, minced; 0.5 cup Italian-seasoned bread crumbs with Romano cheese; 3 sprigs Italian parsley, finely chopped; 1 large egg; 1 teaspoon Italian seasoning; 11 ounces ground pork; 18 ounces lean ground beef; 0.25 cup tomato sauce; 1 teaspoon Dijon mustard (such as Maille®); 1 teaspoon dark brown sugar; 1 teaspoon white vinegar