Preheat oven to 400°F and line a baking sheet with parchment paper.: As the oven warms, you will notice a faint, dry heat that readies the pan to finish the meatballs gently, preserving juiciness. Preparing the parchment reduces sticking and makes cleanup simple. A common mistake is skipping the parchment then tearing the meatballs when removing them, so line the sheet and let the oven reach full heat before baking.
In a large bowl, combine ground beef, egg, breadcrumbs, Parmesan, garlic, oregano, salt, and pepper. Mix until just combined.: You will feel the mixture come together under your hands, slightly tacky but not wet. The aromas of garlic and Parmesan cheese will be noticeable immediately, promising savory depth. Mixing too long creates dense meatballs, so stop once the ingredients are evenly distributed to keep them tender.
Divide the mixture into 16 equal portions. Flatten each portion, place a cube of mozzarella in the center, and wrap the meat around the cheese, forming a ball. Place them on the baking sheet for the time being.: The surface will be cool and soft as you shape each ball, and sealing the edges tightly prevents cheese leakage during cooking. If a seam opens, seal it by pressing gently; otherwise the mozzarella can ooze out while cooking. A frequent error is making portions uneven, which leads to inconsistent cooking, so take a moment to portion evenly.
Heat olive oil in a skillet over medium heat. Brown the meatballs on all sides, then return them to the baking sheet.: When the meatballs hit the pan you should hear a confident sizzle as the exterior begins to caramelize, producing a deep browned crust that enhances flavor. The smell of toasted meat and melted fat will fill the kitchen. Avoid crowding the skillet which causes steaming instead of browning; work in batches if needed for an even crust.
Bake in the preheated oven for 10-15 minutes, or until meatballs are cooked through.: In the oven the caramelized exterior softens slightly while the interior finishes cooking, and you may see juices bead on the surface. The mozzarella should be melted inside without escaping, creating that molten center. Overbaking dries the meatballs, so check for an internal temperature and remove as soon as they are cooked through.
Garnish with fresh parsley and serve with marinara sauce if desired.: The final touch of chopped parsley adds a bright scent and a clean, herbal note that contrasts the richness. Serve warm so the cheese pull is at its best, and if you use marinara, the acidity will cut through the fattiness. A common oversight is letting the meatballs sit too long before serving, which cools the center and diminishes the cheese pull, so plate and serve promptly.