Trim as much fat as possible from ribs. On bone side, work knife tip beneath membrane that covers bone until finger can be work under rack memebrane, loosening enough to get firm grip (try papper towel to hold membrane). Then peel membrane off rack. With knife scrape any fat away from bone. Sprinkle ribs on both sides with even, light coat of salt and pepper. Place ribs, bone side down, on grill. Grill over low fire for 1 1/2 hours, turning every 15-20 min. Baste with apple juice at every turn during the first half of cooking period, either by brushing on or simply sparying apple juice from sray bottle. If smoking with indirect heat, turn at 1 hour intervals, cooking 4-6 hours. Combine vinegar, onion, garlic, mustard, sugar and hot sauce in blender and blend till smooth. Place in saucepan and add ketchup. Simmer 20min. Add lemon slices. Stir occasionally to keep from sticking. Use sauce as frequent baste for last half of cooking period. Becareful not to burn ribs (sugar has tendancy to burn. Serve remaining sauce on side.