1. Preheat broiler.
2. Place poblano chiles and jalapeno peppers on a foil-lined baking sheet. Broil 10 minutes or until blackened and charred, turning occasionally. Place peppers in a paper bag; fold to close tightly. Let stand 15 minutes. Peel chiles and peppers; cut in half lengthwise. Discard seeds and membranes. Chop poblano chiles. Finely chop jalapeno peppers.
3. Combine poblano chiles, jalapeño peppers, tomatillos, and next 4 ingredients (through green chiles) in a large bowl.
4. Sprinkle chicken with cumin and pepper. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add half of chicken to pan. Cook 2 1/2 minutes on each side or until browned. Place chicken in a 6-quart electric slow cooker. Repeat procedure with remaining chicken. Pour tomatillo mixture over chicken. Cover and cook on LOW for 3 1/2 hours or until chicken is tender.
5. Remove chicken from slow cooker; keep warm. Pour sauce into a medium saucepan. Bring to a boil; reduce heat, and simmer, uncovered, 25 minutes or until reduced to 4 1/2 cups.
6. Serve chicken with sauce. Sprinkle with chopped cilantro. Garnish with sour cream, if desired.