Heat 1-2 tablespoons olive oil in your large skillet over medium heat. Add fresh baby spinach and cook for 2-3 minutes until wilted. Add 3 chopped garlic cloves in the last minute, stirring until fragrant. Remove from heat and let cool completely before chopping finely.
Preheat your oven to 350°F. In a large bowl, combine ground beef, breadcrumbs, eggs, milk, remaining garlic, Parmesan, salt, and pepper. Mix gently with clean hands until just combined - overmixing makes tough meatballs. Fold in the cooled spinach mixture until evenly distributed throughout.
Roll the mixture into golf ball-sized portions. Press a mozzarella cube into the center of each ball, sealing the meat completely around it. Make sure no cheese shows through the meat surface. Place finished meatballs on a plate as you work through the mixture.
Heat 2 tablespoons olive oil in your oven-safe skillet over medium heat. Add meatballs in batches, avoiding overcrowding the pan. Sear for 6-8 minutes, turning occasionally until golden brown on all sides. This step locks in flavor and creates appealing color.
Pour marinara sauce over and around the seared meatballs in the skillet. Cover tightly with aluminum foil and transfer to preheated oven. Bake for 15-20 minutes until internal temperature reaches 165°F and cheese centers are completely melted and gooey.