In a skillet, melt butter with olive oil over medium heat.
Add sliced onions and cook slowly 15–20 minutes until deeply caramelized and golden.
Place the meatballs in the slow cooker.
Top with the caramelized onions.
In a bowl, mix beef broth, Worcestershire sauce, garlic powder, thyme, salt and pepper.
Pour evenly over the meatballs.
Cover and cook on LOW for 4–6 hours until meatballs are tender and infused with flavor.
Sprinkle shredded cheese over the top during the last 20 minutes.
Cover and let melt until bubbly and slightly golden.
Finish with fresh chopped parsley before serving.
