In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, minced garlic, egg, salt, black pepper, oregano, and red pepper flakes. Mix until well combined.
Shape the mixture into meatballs, about 1 to 1.5 inches in diameter. You should have around 24 meatballs.
For added texture, you can place the meatballs on a baking sheet and broil them in the oven for 5-7 minutes until slightly browned. This step is optional but enhances the flavor.
Place the meatballs in the crockpot and pour the marinara sauce over them, making sure they are well-coated.
Cover the crockpot and cook on low for 6 hours or on high for about 3 hours, until the meatballs are cooked through and tender.
Once cooked, give the sauce a gentle stir before serving.
Serve the meatballs over spaghetti, in hoagie rolls, or alone as an appetizer with toothpicks.