Trim the slab of ribs, removing the brisket, skirt, fat and smaller ribs on the thin/bottom side of the slab. Set aside.
Stir together the granulated garlic and onion, pepper and salt in a bowl. Thoroughly cover the slab with the dry rub seasoning mix, leaving no piece of it uncoated. Place in the refrigerator to dry brine for 3 to 5 days.
Place the ribs on a cold smoker as far away from the firebox as possible, with the thinnest side of the slab facing away from the fire. Start the fire and maintain 200 degrees F and a clean burning fire for 3 hours. (No super thick cloudy smoke. Thin blue smoke is best.)
Increase the temperature to 275 degrees F and continue to cook until the ribs have a deep and rich mahogany color, slightly crispy edges, small pools of liquified fat on the surface and the slab is loose and bending, 30 to 45 minutes. Remove from the smoker and wrap in foil; return to the smoker for 30 minutes. Remove and rest for 20 to 30 minutes.