For the basic BBQ rub: Using a coffee grinder, grind the turbinado sugar with a few "pulses" until it is lightly powdered. Combine with the granulated sugar, salt, onion powder, granulated garlic, cayenne, black pepper, mustard, chili powder, cumin, and paprika in a bowl and hand stir until well incorporated. Store in an airtight container.
For the BBQ mother sauce: In a medium saucepan, heat the oil over medium heat. Add the onion and saute until translucent, about 5 minutes. Turn the heat on low. As the onion is getting close, add the garlic and cook until lightly golden, about 2 minutes longer. Add the ketchup, honey, tomato paste, vinegar, brown sugar, Worcestershire, dry mustard, cayenne and black pepper and stir well.
Slowly add up to 1/2 cup water until the sauce reaches the consistency you like. A slightly thick consistency is best. Add about 3 tablespoons of the basic BBQ rub, stir well and taste. The sauce should have a good, well-rounded flavor. Add more of the basic BBQ rub in 1-tablespoon increments until your desired flavor is achieved, up to about 1/2 cup total. Cool and store in an airtight container in the refrigerator for up to 10 days.
For the ribs: Rinse the ribs and remove the membrane from the back. Trim any excess fat from the tops of the slabs. Trim 1 bone from the large end of the ribs and 2 bones from the small end. This will give you a much more consistent slab for cooking.
Starting on the backs, sprinkle the slabs of ribs with approximately 1/2 tablespoon of the basic BBQ rub each, then add 1/2 tablespoon yellow mustard each and massage into the meat. Flip the ribs over and repeat. Wrap tightly with plastic wrap and refrigerate for at least 8 hours. For a contest, I marinate ribs like this for 12 to 16 hours.