Black Bean & Sweet Potato Tostadas

Preheat broiler. Place sweet potato in a microwave-safe bowl; microwave, covered, on high until tender, 2-3 minutes. For sauce, mix mayonnaise, lime juice and chipotle pepper., In a large saucepan, heat oil over medium heat; saute onion until tender, 3-4 minutes. Add garlic and seasonings; cook and stir 1 minute. Stir in beans; heat through. Stir in sweet potato; keep warm., In 2 batches, spritz both sides of tortillas with cooking spray and place on a baking sheet; broil 4-5 in. from heat until crisp and lightly browned, about 1 minute per side. Sprinkle immediately with cheese. To serve, top tortillas with sauce, bean mixture and, if desired, lettuce.