Chipotle Chicken Salad

INGREDIENTS

• 1 medium white onion, chopped
• 1/3 cup olive oil
• 2 firm-ripe California avocados, halved, pitted, and left unpeeled
• 1 heart of romaine, separated into leaves
• 1/2 cup packed fresh cilantro sprigs
• 1 rotisserie-cooked chicken (2 lb) at room temperature
• 3/4 teaspoon black pepper
• 1 rounded tablespoon canned chipotles in , or to taste
• 3/4 teaspoon salt
• 1/3 cup fresh lime juice
• 1 (15- to 19-oz) can black beans, rinsed and drained
• 3 oz corn chips such as Fritos (1 1/2 cups)