1. Preheat grill to medium-high. Grill sweet peppers and red onion rings on all sides until grill marks appear, about 2 minutes per side. Transfer to plate and let cool. Discard pepper stems and seeds, and chop peppers and onion into small dice.
2. Combine beans, avocado, cilantro, sweet peppers, and red onion in salad bowl and toss with lime juice, oil, salt, and pepper.