Preheat the oven to 350°F (175°C). Grease a loaf pan with nonstick spray, butter, or a drizzle of olive oil.

Heat 1 tablespoon olive oil in a medium skillet over medium heat. Add the chopped onion, green pepper, mushrooms, and minced garlic. Sauté for 5–7 minutes until softened and fragrant. Remove from heat and let cool slightly.

In a large bowl combine the ground beef, milk, breadcrumbs, egg, salt, pepper, garlic powder, onion powder, Adobo seasoning, Worcestershire sauce, and the sautéed vegetables. Use clean hands and mix until just combined; do not overwork.

Place half the meat mixture into the prepared loaf pan. Press it down evenly into a flat layer.

Arrange 4 very thick slices of mozzarella evenly over the first meat layer.

Spread the remaining meat mixture on top of the cheese, pressing gently to seal the edges. Reserve the last 4 slices of mozzarella for the finish.

Bake for 30 minutes on the middle rack.

Remove from the oven. Place the remaining 4 mozzarella slices on top and sprinkle with chopped parsley if desired. Return to the oven for 10–15 minutes, or until the top cheese is melted and bubbly with slightly crispy edges.

Let the meatloaf rest for 5–10 minutes before slicing. Serve warm.