Squeeze juice from lemons into blender or food processor. Slice lemon rinds; set aside. Add green pepper, onion and celery to blender. Whirl to puree.
Transfer mixture to 3-quart saucepan. Stir in tomato puree, sugar, hot sauce, red-pepper flakes and reserved lemon rinds. Simmer, covered, 20 minutes. Discard rinds. Sauce may be refrigerated for up to several weeks. Makes about 1 quart.
Combine salt, black pepper and red-pepper flakes. Rub spareribs in large glass baking dish. Pour vinegar over ribs. Refrigerate, covered, at least 4 hours or overnight. Remove ribs from marinade and place in shallow roasting pan. Roast in 350 degree F oven for 1-1/2 hours. Increase oven temperature to 400 degrees F. Roast 30 minutes. During last 15 minutes of cooking, baste with 2 cups of the barbecue sauce. Makes 12 servings.