01 - Preheat oven to 375°F. Line a loaf pan with parchment paper or grease lightly with olive oil.
02 - Heat a splash of olive oil in a skillet over medium heat. Sauté the diced onion for 3 to 4 minutes until softened. Add 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, mozzarella, Parmesan, beaten eggs, breadcrumbs, milk, marinara sauce, Italian herbs, salt, black pepper, and red pepper flakes. Mix gently until just incorporated, 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, even layer of marinara sauce across the top.
05 - Bake for 55 to 65 minutes, or until the internal temperature registers 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 additional warmed marinara sauce on the side.