In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, grated Parmesan cheese, minced garlic, salt, black pepper, thyme, and red pepper flakes. Mix until just combined; do not overmix.
Form the mixture into meatballs, about 1.5 inches in diameter. Place the meatballs in the slow cooker.
In a separate bowl, mix the beef broth, Worcestershire sauce, and soy sauce. Pour this mixture over the meatballs in the slow cooker.
Layer the sliced onions over the meatballs.
Cover and cook on low for 6 hours, or on high for 3 hours, until the meatballs are cooked through and tender.
About 15 minutes before serving, remove the lid and sprinkle Gruyère cheese over the top of the meatballs. Cover again and allow the cheese to melt.
Once done, serve the meatballs hot, garnished with chopped parsley. Enjoy!