Preheat and Prepare: Preheat your oven to 375°F. In a large bowl, mix the ground meat with onion, garlic, egg, breadcrumbs, milk, salt, pepper, oregano, and basil until just combined. Avoid overmixing for a tender texture.
Make the Cheese Sauce: In a small saucepan over medium heat, whisk together milk and flour until smooth. Slowly stir in the cheddar and Parmesan cheeses until melted and creamy. Add salt, paprika, and ground mustard. Stir continuously until the sauce thickens, then mix in chopped spinach if using.
Combine Pasta and Sauce: In a separate bowl, gently toss the cooked macaroni with the cheese sauce until all pasta is evenly coated.
Assemble the Meatloaf: On a lined baking tray, shape the meat mixture into a rectangular base about 1 inch thick. Spread the cheesy macaroni evenly over the meat layer, leaving a small border. Carefully fold the edges of the meat over the pasta to encase the filling, ensuring an even distribution.
Bake the Meatloaf: Transfer the assembled loaf into the preheated oven and bake for about 50 to 60 minutes, or until the meat is fully cooked. For an added crunch, combine extra cheddar, panko breadcrumbs, and parsley; sprinkle this mixture over the top during the last 10 minutes of baking.
Rest and Serve: Once cooked, let the meatloaf rest for 5 minutes before slicing. This step allows the juices to settle, making each slice succulent and packed with flavor.