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

Cook the vegetables: In a medium skillet, heat olive oil over medium heat. Add the onions, peppers, mushrooms, and minced garlic. Sauté until softened and fragrant, about 5–7 minutes. Remove from heat and let cool slightly.

Mix the meatloaf base: In a large mixing bowl, combine ground beef, milk, breadcrumbs, egg, salt, pepper, garlic powder, onion powder, Adobo seasoning, Worcestershire sauce, and the sautéed vegetables. Use clean hands to mix until just combined—avoid overmixing.

Assemble the first layer: Place half of the meat mixture into the prepared loaf pan, pressing it down gently into an even layer.

Add the cheese filling: Layer 4 very thick slices of mozzarella cheese evenly over the meat mixture.

Top with the remaining mixture: Spread the other half of the meat mixture over the cheese, pressing gently to seal the edges. Reserve the last 4 slices of cheese for topping.

Bake: Place the loaf pan in the oven and bake for 30 minutes.

Add the topping: Remove from the oven, place the remaining 4 slices of mozzarella on top, and sprinkle with parsley if desired. Return to the oven for another 10–15 minutes, or until the cheese is melted and bubbly (slightly crispy edges are delicious).

Rest and serve: Allow the meatloaf to rest for 5–10 minutes before slicing. Serve warm and enjoy!