Rub the meat completely with the mustard. And spices to a large zip-lok bag, seal and shake until mixed. Place meat in bag and coat with the rub. Refrigerate overnight.
In a two quart saucepan melt butter, add the onions and garlic. Saute for five minutes then add lemons, limes and beer. When the foam subsides add remaining ingredients and simmer for 20-25 minutes.
Prepare the grill for the indirect heat method and remove the meat from the bag and place on the grid. Smoke for 25 minutes at 250 degrees and start basting with the sop every 25 minutes. Smoke for 2-3 hours or until internal temperature reaches 155 degrees, then wrap in foil and finish cooking until internal temperature reaches 185 degrees.
Remove from grill and let meat cool for 20 minutes prior to slicing. Serve BBQ sauce on the side.