Preheat the oven to 350°F. Place the oven rack in the middle so the loaf bakes evenly.

In a large mixing bowl, combine ground beef, breadcrumbs, eggs, milk, grated onion, minced garlic, Worcestershire sauce, Italian seasoning, salt, and pepper. Mix just until evenly combined with no dry breadcrumb pockets.

Lay plastic wrap or parchment on a flat surface and press the meat mixture into a 10x12 inch rectangle. Press firmly to form a cohesive layer with even thickness.

Layer the wilted spinach, sliced roasted red peppers, and shredded mozzarella over the surface, leaving a 1-inch border. Distribute so the filling can create a continuous spiral when sliced.

Roll tightly into a log using the plastic wrap, then seal the ends and place seam-side down in a loaf pan. Press lightly as you roll so the filling stays inside.

Spread the glaze (ketchup and brown sugar) over the top of the meatloaf. Make sure it reaches the corners for a caramelized finish.

Bake at 350°F for 60–70 minutes until the internal temperature reaches 160°F. You should see the glaze set and lightly darken, and juices should run clear when tested.

Rest the stuffed meatloaf for 10 minutes before slicing. This helps the spiral hold together so the melted cheese and peppers stay visible in the cut.