Prepare the Meatballs: In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, milk, egg, minced garlic, salt, and black pepper. Mix well until all the ingredients are fully incorporated.

Shape the Meatballs: Roll the mixture into 1-inch balls and place them onto a baking sheet. This should make about 20-24 meatballs.

Sear the Meatballs: In a large skillet, heat some oil over medium heat. Add the meatballs in batches and sear until they are browned on all sides. This helps lock in the juices.

Prepare the Sauce: In the slow cooker, mix together the marinara sauce, beef broth, Worcestershire sauce, oregano, basil, and red pepper flakes.

Slow Cook: Carefully add the browned meatballs into the slow cooker, making sure they're fully submerged in the sauce.

Cook: Set the slow cooker to low and cook for 6-7 hours, or on high for 3-4 hours, until the meatballs are fully cooked and tender.

Serve: Once done, give the sauce a good stir. Serve the meatballs hot, garnished with additional parsley and Parmesan if desired.