In a medium bowl, combine the 2 cups of shredded or diced cooked chicken with 1/2 cup of your favorite BBQ sauce. Stir until the chicken is completely coated. For extra flavor, add a teaspoon of chili powder or hot sauce if desired.

Place a large nonstick skillet or cast iron skillet over medium heat. Add about a teaspoon of butter or a light spray of cooking oil. Ensure the pan is hot enough to sizzle but not burn the tortilla.

Place one flour tortilla flat into the heated pan. Sprinkle one-third of a cup of shredded cheese evenly over the tortilla. On one half, spoon about 1/4 of the BBQ chicken mixture, then sprinkle 1 tablespoon of diced red onion. Top the filling with another one-third cup of cheese.

Fold the empty side of the tortilla over the filling to create a half-moon. Press down gently with a spatula. Cook for 2-3 minutes until golden brown.

Carefully flip the quesadilla and cook another 2-3 minutes, pressing occasionally, until both sides are crispy and cheese is melted.

Transfer to a cutting board, let rest 1 minute, then slice into wedges. Garnish with cilantro if using. Repeat with remaining ingredients.