To make the meatballs: In a large bowl, combine the beef, parsley, egg, garlic, salt, and red pepper flakes until mixed well. Form into 24 meatballs and place in slow cooker.
Make sauce: In another large bowl, stir together crushed tomatoes, tomato paste, onion, oregano, and garlic and season with salt and pepper. Pour over meatballs.
Cook, covered, on low until meatballs are cooked through, 5 to 5 1/2 hours.