Place the lettuce in a deep 3-quart serving bowl. Layer black beans, red onion, drained chile peppers, sweet pepper, and cilantro on top of lettuce.
For dressing, in a small bowl stir together sour cream, lime juice, chili powder, salt, and garlic powder.
Cover salad and dressing; chill separately for at least 2 hours or up to 24 hours.
Drizzle dressing over salad; gently toss salad to coat vegetables with dressing. Sprinkle with chopped tomato. Makes 8 to 12 servings.