1. In a large bowl, combine flour, salt and pepper. Toss ribs in seasoned flour, shaking off excess.
2. Warm oil in a large skillet over medium-high heat. Working in batches, cook ribs, turning with tongs, until browned on all sides, about 10 minutes. Transfer ribs to slow cooker. Pour off all but 1 Tbsp. fat from skillet. Add onion, ginger and garlic to skillet. Cook, stirring often, until slightly softened, 2 to 3 minutes. Pour in broth, increase heat to high and bring to a boil, stirring with a wooden spoon to pick up any browned bits on bottom of skillet. Stir in hoisin sauce, soy sauce and five-spice powder until well combined. Pour contents of skillet over ribs in slow cooker.
3. Cover and cook until beef is very tender, with slow cooker on low for 6 to 8 hours or on high for 3 to 4 hours. Remove ribs to a platter. Skim as much fat as possible from cooking liquid in slow cooker; discard fat. Spoon remaining liquid over ribs and serve hot.