01 - Preheat the oven to 375°F. Line a loaf pan with parchment paper or lightly grease it with olive oil.
02 - In a skillet over medium heat, sauté the diced onion in a splash of olive oil for 3-4 minutes until softened. Add the minced garlic and cook for 30 seconds until fragrant. Stir in the chopped spinach and cook for about 2 minutes until wilted. Remove from heat and let the mixture cool slightly.
03 - In a large mixing bowl, combine the ground beef, cooled spinach mixture, shredded mozzarella, grated Parmesan, beaten eggs, breadcrumbs, milk, marinara sauce, dried Italian herbs, salt, black pepper, and red pepper flakes. Gently mix with your hands or a fork until just combined — avoid overmixing to keep the texture tender.
04 - Transfer the mixture to the prepared loaf pan, pressing gently to shape into an even loaf. Spread a thin layer of additional marinara sauce across the top.
05 - Bake for 55-65 minutes, or until the internal temperature registers 160°F and the loaf is fully set with lightly browned edges.
06 - Remove from the oven and let the meatloaf rest in the pan for 10 minutes before slicing. Serve with extra warmed marinara sauce on the side.