Cut ribs into 2-3 bone portions and lay on a lined baking sheet. Season with salt and pepper and bake for 10 minutes. Turn oven to broil and continue cooking the ribs for 5 minutes on each side until they are lightly browned.
Transfer ribs to the slow cooker.
Combine ketchup, brown sugar, steak sauce, vinegar, salt, smoke flavor, and onion and stir until smooth. Pour over ribs.
Cover slow cooker and cook on LOW for 6-8 hours or until the ribs are very tender and almost fall off the bone.