Season each side of the chicken with the mesquite seasoning and lay flat in a large baking dish.
Drizzle BBQ sauce over the top of each chicken breast and sprinkle with diced onion.
Bake uncovered for 20 minutes, then remove the dish, add cheese and bacon to the top and return to oven for 5-10 minute or until the cheese is melted and the chicken is done (internal temperature of 165˚F.)