Mix the first nine ingredients. Cut roast in half; rub with 1/4 cup spice mixture. Cover and store remaining mixture in a cool, dry place for up to 1 year., Place onions and peppers in a 4-qt. slow cooker. In a large skillet, heat oil over medium heat. Brown roast on all sides. Transfer meat and drippings to slow cooker. Top with salsa. Cook, covered, on low until meat is tender, 8-10 hours., Remove roast; shred with two forks. Skim fat from cooking juices. Return meat to slow cooker; heat through. Using a slotted spoon, place 1/2 cup meat mixture on each tortilla. Sprinkle with cheese. Add toppings of your choice.