Make Meatball Mixture:In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, garlic, egg, parsley, salt, pepper, Italian seasoning, and milk. Mix gently with your hands until well combined—don’t overmix.
Form Meatballs:Roll the mixture into 1 1/2-inch balls and place them on a plate or tray.
Brown the Meatballs:In a large skillet over medium heat, heat olive oil. Add meatballs in batches and cook until browned on all sides, about 5-6 minutes total. Transfer to a paper towel-lined plate.
Simmer in Sauce:In the same skillet, drain excess grease. Pour in marinara sauce and bring to a simmer. Gently add the browned meatballs to the sauce, cover, and let simmer for 20 minutes until fully cooked through.
Serve:Serve hot over spaghetti, in sub rolls, or as an appetizer with extra Parmesan and parsley garnish.