Heat a large cast iron pan over high heat. Meanwhile mix together the spices and rub all over the pork butt. Sear each side of the pork butt in the heated skillet until each side is browned, about 2 minutes per side. Line the bottom of the slow cooker with the onion rings and add the limes and their juice along with ½ cup water. Place the Boston butt on top and cover. Cook on high for 6-7 hours or until the meat is tender. Remove the meat and shred with a fork and place on a rimmed baking sheet. Place under the broiler for 5 minutes or until the edges of the pork just start to crisp. Serve with warm tortillas, cheese and toppings.