Combine the chicken and 1/2 cup of sauce, stirring until evenly combined. Set aside.
To the top of a flour tortilla, add a tablespoon of barbecue sauce and spread it out evenly, but not to the edge.
Spread a 1/4 cup of cheese over half of the tortilla. Top that with a quarter of the chicken mixture. Sprinkle onions and cilantro over top. Spread the remaining 1/4 cup evenly over top. Fold the other half of the quesadillas over top to close it.
Heat a large skillet over medium heat. Once hot, spray liberally with non stick cooking spray. Carefully transfer the stuffed quesadilla to the hot pan. Cook for about 2 minutes, or until golden brown, flip and continue cooking the other side an additional minute or two- until it's also golden brown.
Remove the cooked quesadilla to a cutting board. Let it cool for 1-2 minutes before using a pizza cutter to slice into 3-4 wedges and serving. Repeat for the remaining tortillas/quesadillas.
If wanting to serve all at once, add the cooked quesadillas to a baking sheet and store in a warm oven until they're all done and ready to be sliced/served.