In a large bowl, mix together the ground beef, breadcrumbs, chopped onion, minced garlic, egg, Worcestershire sauce, Dijon mustard, thyme, salt, and pepper until just combined.
On a clean surface or parchment paper, spread the meat mixture into a rectangle about ½ inch thick.
In another bowl, combine the mozzarella cheese, diced bell peppers, and grated Parmesan to create the stuffing mixture.
Spread the cheese and pepper mixture evenly over the meat rectangle, leaving a small border around the edges.
Carefully roll the meatloaf from one side to form a log shape, ensuring the stuffing remains inside.
Place the stuffed meatloaf seam-side down in a lined or greased baking dish.
Brush the top with ketchup for a flavorful glaze.
Bake in the preheated oven for 45-55 minutes, or until the internal temperature reaches 160°F (70°C).
Once done, remove from the oven and let it rest for about 10 minutes before slicing.
Garnish with fresh parsley before serving.
Notes
Let the meatloaf rest before slicing for better presentation.