Preheat oven's broiler and set oven rack about 6" from heat source.
Set slow cooker on Medium, add sliced onions and cover.
Place ribs on baking sheet, season with salt and ground black pepper, and place under preheated broiler until brown, about 5 minutes.
Place browned ribs into preheated slow cooker with onion. Cover and cook 4 hours.
After 4 hours, pour entire bottle of barbeque sauce over ribs and onions, pour wine into barbeque sauce bottle, shake and pour into slow cooker to capture all the BBQ goodness. Stir gently to combine wine and sauce. Cover and cook until tender, about another 4 hours.