Adjust oven rack to upper-middle position, and preheat oven to 250.
Mix sugar, paprika, pepper, garlic powder and salt in a small bowl.
If using liquid smoke, mix mustard and liquid smoke in a small bowl.
Brush both sides of each slab with mustard, then sprinkle both sides with the dry rub.
If using just mustard, brush directly on each slab, then sprinkle both sides with the dry rub.
***Line a lg. jelly roll pan or other shallow roasting pan with a sheet of heavy-duty aluminum foil.
Place the pan on the oven floor (if oven is gas), or bottom oven rack (if oven is electric), making sure the foil covers the entire oven level.
Roast ribs until fork-tender- 2-3 hours for spare ribs, and 1 1/2-2 hours for baby-back ribs.
Remove foil-lined pan from oven, and pour off fat. Transfer ribs to foil-lined pan, meat side down. Turn on broiler. Brush ribs with 1/2 the Ah-So Chinese Style Sauce.
Put pan under broiler until glaze bubbles vigorously. Remove pan, turn the ribs, brush with the remaining sauce, and return to the.
broiler until glaze bubbles vigorously. Be careful, because they can burn.
Let stand 5-10 minutes.
Cut into individual servings, and serve.
***Note: My Aunt Ann makes these directly on the racks in her self-cleaning oven, and just put aluminum foil on the bottom rack to catch everything. Once the ribs are made she just soaks the racks to clean, and turns on the automatic cleaning system for her oven.