01 - Preheat the oven to 375°F (190°C). 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, Italian herbs, salt, black pepper, and red pepper flakes. Mix gently until just combined—avoid overmixing to keep the texture tender.
04 - Transfer the mixture to the prepared loaf pan and shape it into an even loaf. Spread a thin layer of extra marinara sauce evenly over the top.
05 - Bake for 55-65 minutes, or until the internal temperature reaches 160°F (71°C) and the loaf is firm and set.
06 - Remove the meatloaf from the oven and let it rest for 10 minutes before slicing. Serve with additional warm marinara sauce on the side if desired.