Place a pizza stone or a round baking sheet in a cold oven. Preheat oven to 425°F, allowing stone or pan to become hot.
In a small skillet, heat oil over medium heat. Add onions and sauté until golden brown (about 8-10 minutes).
Meanwhile, spread 2 tablespoons barbecue sauce on tortilla. Top with chicken; set aside. Add remaining tablespoon of barbecue sauce to onions and cook 1 minute; spread onions over chicken.
Sprinkle with cheese. Carefully transfer pizza to hot stone or baking sheet and bake 6-8 minutes or until cheese is melted and bubbly.