1. Mix onion, bell pepper, beans, chicken, chili powder, garlic, salt and 1/2 cup cheese in a medium bowl.
2. Spread 1/3 of salsa in slow cooker. Layer 6 tortillas on top, overlapping. Top with half of filling, spread with 1/3 of salsa and sprinkle with 1/2 cup cheese. Make a second layer with 6 remaining tortillas. Top with remaining filling and salsa. Reserve remaining 1/2 cup cheese. Cover and cook on low for 2 to 4 hours.
3. During last 10 minutes of cooking, sprinkle with 1/2 cup cheese. Cover; cook until cheese melts. Spoon onto plates, top with sour cream, if desired, and serve.