Stuffed Italian Meatloaf with Cheese and Spinach Layers

Ingredients

• 2 pounds ground beef
• 1 cup Italian-style breadcrumbs
• 2 large eggs
• 1/2 cup whole milk
• 1 small onion, finely chopped
• 2 garlic cloves, minced
• 1 teaspoon kosher salt
• 1/2 teaspoon ground black pepper
• 1 teaspoon Italian seasoning
• 2 cups shredded mozzarella cheese
• 1 cup baby spinach, optional
• 1/2 cup grated Parmesan cheese
• 1 cup marinara sauce, plus extra for serving
• Cooking spray or olive oil for greasing