Rub roast with salt and pepper. Cut small slits in roast; insert garlic slices. Place on rack in broiler pan. Broil for 15 minutes. Place roast in slow cooker. Rinse pinto beans; drain. Add to slow cooker. Add oregano, cumin, chili powder, green chiles and enough water to cover.
Cook, covered, on Low for 8 or 9 hours, stirring occasionally. Add the chipotles. Cook, uncovered, for 1 hour longer. Skim off fat; discard bones. Serve on a bed of corn chips with shredded cheese, chopped green onions, tomatoes, cucumbers, lettuce, avocado, salsa and sour cream. Add a can of diced tomatoes or rotel to leftovers and serve over rice.