Preheat: Heat the oven to 400°F and line a baking sheet with parchment.
Make the panade: Combine the breadcrumbs and milk in a large bowl and let it sit 5 minutes until the milk is fully absorbed.
Mix: Add the ground beef, egg, parmesan, garlic, parsley, salt, and pepper to the soaked breadcrumbs. Mix gently with your hands just until combined, don't overwork it.
Shape: Roll the mixture into about 20 meatballs, roughly 1 1/2 inches each, and place them on a parchment-lined baking sheet.
Bake: Bake at 400°F for 18-20 minutes, until browned and the internal temperature reaches 160°F.
Simmer: Warm the marinara sauce in a large pot or deep skillet, add the baked meatballs, and simmer 10 minutes so they soak up the sauce.
Serve: Serve over cooked spaghetti or piled into sub rolls.
Notes
Soak the breadcrumbs in milk before mixing, that panade is what keeps the meatballs tender. Bake instead of frying for even browning with far less mess.