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 to 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. Mix gently until just combined, being careful not to overwork the meat.
04 - Transfer the mixture to the prepared loaf pan and shape it into an even loaf. Spread a thin layer of additional marinara sauce across the top.
05 - Bake for 55 to 65 minutes, or until the internal temperature reaches 160°F and the loaf is fully set.
06 - Remove from the oven and let the meatloaf rest for 10 minutes before slicing. Serve with extra marinara sauce on the side if desired.