01 - Preheat oven to 400°F. Line a baking tray with parchment paper or grease lightly.
02 - Combine breadcrumbs and milk in a large bowl. Let sit for 2 minutes to absorb liquid.
03 - Add ground beef, chopped onion, minced garlic, egg, Parmesan, parsley, oregano, basil, salt, pepper, and red pepper flakes if using. Mix gently until just combined; avoid overmixing.
04 - With wet hands, shape mixture into sixteen golf ball-sized meatballs and place on prepared tray.
05 - Heat olive oil in a large oven-safe skillet over medium heat. Brown meatballs in batches, turning to color all sides evenly, about 5 to 6 minutes total.
06 - Return browned 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 cooked through, reaching internal temperature of 160°F.
08 - Serve meatballs hot with extra sauce, over pasta, or with crusty bread.