Add the butter, diced onions, and garlic to a pan over medium-high heat and cook for 3-4 minutes, until translucent. Transfer everything to the slow cooker.
Add the cube steak and sear the meat for 5 minutes on each side until browned. Add the beef broth and gravy mix, then season with a pinch of salt and pepper. Whisk until well combined, then cover the slow cooker and cook on the "high" setting for 3 hours, or on "low" setting for 6 hours.
Once the meat is tender, serve over mashed potatoes or white rice, and garnish with fresh chopped parsley.