In a small bowl, combine cumin, salt, and pepper; set aside. Trim fat from meat. Cut meat into 1-inch pieces. Sprinkle cumin mixture over meat. Coat a large skillet with cooking spray. Cook half of the meat in hot skillet over medium heat until brown. Remove meat from skillet. Add oil to skillet. Brown remaining meat in hot oil. Drain off fat. Place meat in a 3-1/2- or 4-quart slow cooker. Add tomatillos, onion, 1 teaspoon of the lime peel, lime juice, and garlic. Stir to combine.
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours.
If using low-heat setting, turn to high-heat setting. Add sweet pepper to cooker. Cover and cook for 15 minutes more. Fill corn tortillas with meat mixture; sprinkle with cilantro and remaining lime peel. If desired, serve with green salsa, if desired. Makes 6 servings.