1. Preheat broiler to high.
2. Halve, stem, and seed chiles. Place chiles, skin side up, on a foil-lined baking sheet coated with cooking spray; broil for 5 minutes or until charred. Place chiles in a paper bag; seal. Let stand for 15 minutes. Peel and discard skins. Arrange tomatillos on prepared baking sheet, and broil 14 minutes or until blackened, turning once. Combine the chiles, tomatillos, 1/4 cup cilantro, cumin, and oregano in a blender. Add 1 cup broth; process until smooth.
3. Heat a large Dutch oven over medium-high heat. Add 2 teaspoons olive oil; swirl to coat. Add onion, carrot, celery, and bell pepper; sauté for 2 minutes, stirring occasionally. Stir in flour; sauté for 2 minutes, stirring frequently. Add garlic; sauté for 30 seconds, stirring constantly. Place onion mixture in a large bowl.
4. Sprinkle chicken with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Add 2 teaspoons oil to pan; swirl to coat. Add half of chicken; sauté 3 minutes. Add browned chicken to onion mixture. Repeat procedure with remaining chicken and 2 teaspoons oil. Combine remaining 1 cup broth, tomatillo mixture, onion mixture, and hominy in pan over medium-high heat, and bring to a boil. Cover, reduce heat, and simmer for 45 minutes, stirring occasionally. Stir in 1/2 teaspoon salt and 1/4 teaspoon black pepper. Ladle 1 2/3 cups stew into each of 6 bowls, and top each with 1 tablespoon sour cream. Garnish with cilantro, if desired.