Cut ribs into 3- or 4-rib serving-size portions. Place in bowl of a slow cooker. Sprinkle with salt and pepper, turning to evenly coat. Add half of chopped celery and half of onion. Cover and cook on high setting 5 to 6 hours or low setting 7 to 8 hours, until tender. Turn meat halfway through.
Before serving, melt butter in a medium-size saucepan over medium heat. Add remaining celery and onion. Stir often until soft, about 5 minutes. Whisk in vinegar, ketchup, sugar, Worcestershire, mustard and chili flakes. Remove seeds from lemon slices and add slices. Bring to boil, then reduce heat and simmer, stirring often, about 5 minutes. Or make sauce ahead and refrigerate up to 2 days. Reheat before using. Place ribs on a large platter or plates. Drizzle with warm sauce. Terrific with mashed or baked potatoes.