01 - Preheat oven to 375°F (190°C). Line a loaf pan with parchment paper or lightly grease it.
02 - In a skillet over medium heat, sauté the onion in a splash of olive oil for 3-4 minutes until softened. Add minced garlic and cook for 30 seconds until fragrant. Add chopped spinach and cook until wilted, about 2 minutes. Remove from heat and let cool slightly.
03 - In a large mixing bowl, combine ground beef, cooled spinach mixture, shredded mozzarella, grated Parmesan, lightly beaten eggs, breadcrumbs, milk, marinara sauce, Italian herbs, salt, pepper, and red pepper flakes. Mix until just combined — do not overwork the meat.
04 - Transfer the mixture to the prepared loaf pan and shape into an even loaf. Spread a thin layer of extra marinara sauce across the top.
05 - Bake for 55-65 minutes, or until the internal temperature reaches 160°F (71°C) and the loaf is fully set.
06 - Let the meatloaf rest for 10 minutes before slicing to allow juices to redistribute. Serve with additional warmed marinara sauce if desired.