**Prepare the Vegetable Mixture:**
- Heat olive oil in a skillet over medium heat.
- Add onions, bell peppers, and mushrooms. Sauté for about 10 minutes until softened and caramelized.
- Season with salt, pepper, and garlic powder. Set aside to cool.
**Mix the Meatloaf Base:**
- In a large mixing bowl, combine ground beef, egg, breadcrumbs, milk, Worcestershire sauce, garlic powder, onion powder, salt, and pepper.
- Mix gently to avoid overworking the meat.
**Layer the Meatloaf:**
- Preheat oven to 350°F (175°C).
- Press half the meat mixture into a greased loaf pan.
- Add a layer of 4 slices of provolone cheese, followed by the sautéed vegetable mixture.
- Cover with the remaining meat mixture, sealing the edges.
**Bake the Meatloaf:**
- Place in the oven and bake uncovered for 50 minutes.
- Remove from oven, top with remaining 4 slices of provolone cheese, and bake for another 10 minutes.
- Use a meat thermometer to check for an internal temperature of 160°F.
**Prepare the Sauce:**
- While the meatloaf bakes, mix ketchup, beef broth, brown sugar, Dijon mustard, and garlic powder in a small bowl.
- Brush over the meatloaf in the last 15 minutes of baking for a flavorful glaze.
**Rest and Serve:**
- Let the meatloaf rest for 10-15 minutes before slicing.
- Serve with mashed potatoes, roasted vegetables, or in a sandwich on a hoagie roll.