01 - Preheat oven to 400°F. Line a baking tray with parchment paper or lightly grease it.
02 - In a large bowl, combine breadcrumbs and milk. Let stand for 2 minutes to soften.
03 - Add ground beef, onion, garlic, egg, Parmesan, parsley, oregano, basil, salt, pepper, and red pepper flakes (if using). Mix gently until just combined, avoiding overworking.
04 - With wet hands, shape mixture into sixteen golf ball–sized meatballs and arrange them on the prepared tray.
05 - Heat olive oil in a large oven-safe skillet over medium heat. Brown meatballs in batches, turning to color all sides, approximately 5 to 6 minutes total.
06 - Return meatballs to skillet or transfer to baking dish. Pour marinara sauce over them and place in oven.
07 - Bake for 15 to 18 minutes until meatballs reach an internal temperature of 160°F.
08 - Serve hot with additional sauce, over pasta or alongside crusty bread.