Preheat oven to 400°F. Grease a 10-inch oven-safe skillet with olive oil.
In a bowl, combine the diced chicken, white beans, salt, pepper, and BBQ sauce.
Layer 4 tortillas in a circle around the skillet, allowing them to hang over the edges by about a third. Place 1 more tortilla flat in the center.
Spread half of the mozzarella over the bottom tortilla layer. Top with the chicken and bean mixture, sliced red onion, and the remaining mozzarella.
Add the final tortilla on top, then fold the overhanging tortilla edges up and over the filling.
Brush or spray the top with olive oil, then place a heavy, oven-safe pot (a Dutch oven works great) on top to weigh it down.
Bake for 15 minutes, then remove and carefully flip the quesadilla. Bake for an additional 10 minutes, or until golden and bubbly.
Let cool for 5–10 minutes before slicing into wedges.
Sprinkle with cilantro or green onion and serve with ranch and extra BBQ sauce for dipping.
