Spray large baking sheet with non-stick baking spray. Place flatbreads on baking sheet. On each flatbread, evenly spread 2 tablespoons barbecue sauce.
Sprinkle both cheeses over flatbreads. Mix remaining barbecue sauce with grilled chicken and place on pizzas. Top with diced onion. Bake 5 minutes. Sprinkle cilantro on top and serve.
