
Season the meat with salt and pepper on all sides of the London broil. Set aside.
In a small bowl, stir together red wine vinegar, ketchup, brown sugar, garlic, Worcestershire sauce, and beef broth. Set the sauce aside.
In a cast iron skillet, melt the butter and heat the olive oil. Sear the meat in the pan for about 20 seconds on each side of the meat. Remove the meat and place in a slow cooker. Place the onions on top of the meat and pour the sauce over the meat and onions. Cook on low for 7-8 hours or on high for 3-4 hours. Cook until the meat is very tender and falls apart. Serve immediately.