Season ribs with salt and pepper. Heat oil in a large skillet over medium-high heat and add half of ribs. Cook, turning often, until browned on all sides, about 10 minutes. Transfer to a slow cooker. Repeat with remaining ribs.
Add onion, ginger and garlic to skillet. Cook over medium heat, stirring, until onion is slightly softened, 3 to 5 minutes. Stir in flour and cook for 2 minutes. (Mixture will be very dry.) Add soy sauce and broth. Increase heat to high and bring to a simmer, stirring to pick up browned bits. Pour over ribs and cook on low for 6 to 8 hours or on high for 4 to 6 hours.
Transfer ribs to a platter and cover to keep warm. Strain cooking liquid into a saucepan and bring to a boil over high heat. Boil until liquid has thickened slightly, 10 to 15 minutes. Season with salt and pepper. Ladle sauce over ribs. Serve hot with vegetables and steamed white rice, if desired.