In a large skillet over medium heat, add 1 tablespoon of olive oil. Sauté the diced yellow onion, green bell pepper, and red bell pepper for about 5-7 minutes until they soften and become fragrant. Stir in the minced garlic and cook just until fragrant. Remove from heat and allow to cool slightly.

Preheat your oven to 375°F (190°C). In a large mixing bowl, combine the ground beef, breadcrumbs, eggs, milk, Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Mix gently until just combined.

On a parchment-lined baking sheet, shape half of the meat mixture into a loaf. Layer the cooked onion and bell pepper mixture and 6 slices of provolone cheese in the center. Top with the remaining meat mixture, sealing the edges well.

Place the assembled meatloaf in the preheated oven and bake for 50-60 minutes. Use a meat thermometer to check that the internal temperature reaches 160°F (71°C). Add the remaining 3 slices of provolone cheese on top in the last 10 minutes.

Once baked, remove from the oven and let rest for 10 minutes. Slice with a sharp knife, and garnish with fresh parsley if desired.