Make meatballs: In a large bowl, mix together beef, parsley, egg, garlic, salt, and red pepper flakes until combined. Form mixture 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.
Garnish with parsley before serving.