In a large bowl, combine ground beef, French onion soup mix, panko, milk, egg, and Parmesan. Form into meatballs.

Heat olive oil in a skillet over medium heat. Brown meatballs on all sides, then transfer to the slow cooker.

In the same skillet, cook onions until caramelized, about 10-15 minutes. Add to slow cooker on top of meatballs.

Mix beef broth, red wine if using, and Worcestershire sauce. Pour over meatballs and onions in the slow cooker.

Set slow cooker to low for 6-8 hours or high for 3-4 hours.

Season with salt and pepper.

Sprinkle with fresh thyme before serving.