Remove the thin white membrane off of the bone-side of the ribs. Mix together the brown sugar, dry mustard, cayenne, paprika, garlic powder, onion powder, salt and pepper in a small bowl. Massage the rub into the ribs and let sit for 1 hour or up to overnight.
If cooking on the grill, place the ribs meat-side down next to medium-hot coals that are about 225 degrees F. The indirect heat will cook them slower, making them tender. Allow to cook for 1 hour. Turn ribs every half hour and baste with the Kansas City Barbeque Sauce. Cook until the ribs are tender, about 3 to 4 hours.
If cooking indoors, place in a roasting pan with a rack. Slather the ribs with the Kansas City Barbeque Sauce and tent a piece of aluminum foil over them. In a preheated 350 degrees F. oven, place the ribs, basting with the sauce every 30 minutes and removing the foil for the last 30 minutes and cooking until fork tender, about 2 1/2 to 3 hours.
In a small sauce pot on medium-high heat, heat the oil and add the onion, cooking until translucent.
Add the remaining ingredients in a large bowl and mix together. Add to sauce pan and let simmer for 30 to 45 minutes. Use to baste the pork spare ribs.