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 whole milk. Let soak for 2 minutes.
03 - Add ground beef, chopped onion, minced garlic, egg, Parmesan, parsley, oregano, basil, salt, black pepper, and optional red pepper flakes. Mix gently until just combined without overworking.
04 - With wet hands, shape mixture into sixteen golf ball–sized portions 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 frequently to evenly color all sides for approximately 5 to 6 minutes.
06 - Return all browned meatballs to the skillet or transfer to a baking dish. Pour marinara sauce evenly over them and place in the oven.
07 - Bake for 15 to 18 minutes until meatballs are cooked through and reach an internal temperature of 160°F.
08 - Serve warm with additional sauce over pasta, in sandwiches, or alongside bread.