01 - Preheat oven to 375°F. Line a 9x5-inch loaf pan with parchment paper or lightly grease 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 and translucent. 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 with your hands 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, even layer of extra marinara sauce across the top.
05 - Bake on the center rack for 55 to 65 minutes, until the internal temperature registers 160°F on a meat thermometer and the loaf is firm and set.
06 - Remove from the oven and let the meatloaf rest in the pan for 10 minutes before slicing. Serve with additional warm marinara sauce on the side.