1. Preheat broiler.
2. Flatten poblano pepper with hands. Place on a foil-lined baking sheet; broil 4-6 inches from heat 4 minutes or until blackened and charred. Place in a zip-top heavy-duty plastic bag; seal. Let stand 15 minutes to allow skins to loosen. Peel and discard skins. Coarsely chop poblano pepper.
3. Place poblano pepper and next 10 ingredients (through red onion) in an electric slow cooker. Cover and cook on LOW 6 hours or until vegetables are tender. Uncover, and stir in cilantro, tomato paste, and half of green onions.
4. To serve, spoon chili into six serving bowls; top each serving with 1 tablespoon sour cream, 1 tablespoon green onions, and 2 tablespoons cheese. Serve immediately.