Preheat oven to 400°.
Cut chiles in half lengthwise; discard seeds and membranes. Set chiles aside.
Remove husks from tomatilloes, and chop. Coat a medium nonstick skillet with cooking spray, and place over medium-high heat until hot; add tomatilloes, bell pepper, green onions, and garlic; sauté 3 minutes. Remove from heat; stir in chopped cilantro, chili powder, cumin, and pepper. Set aside.
Place 1/2 cup beans in a medium bowl; mash. Stir in remaining 1/2 cup beans, bell pepper mixture, chicken, corn, sour cream, and 2 tablespoons cheese.
Divide bean mixture evenly among chile halves. Place stuffed chiles in an 8-inch square baking dish. Sprinkle remaining 2 tablespoons cheese over stuffed chiles; bake at 400° for 20 minutes. Serve with flour tortillas and salsa; garnish with cilantro sprigs, if desired.
Note: You can also substitute 1/4 cup chopped drained canned tomatilloes or 1/4 cup chopped green tomato for fresh tomatilloes, if desired.