In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic, salt, and black pepper. Mix until well combined.
Form the mixture into 1-inch meatballs and place them on a baking sheet.
In a separate bowl, whisk together the sweet chili sauce, soy sauce, rice vinegar, sesame oil, and red pepper flakes (if using).
Place the meatballs in the crockpot and pour the sauce mixture over them, ensuring they are well coated.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the meatballs are cooked through and tender.
Serve the meatballs warm, garnished with additional parsley if desired.