In a mixing bowl, combine the shredded cooked chicken with ranch dressing, stirring until the chicken is well-coated. Add crumbled bacon, diced tomatoes, finely chopped red onion, and chopped cilantro to the mixture. Toss everything together gently until the ingredients are evenly distributed.

Take a flour tortilla and place it on a clean work surface. On one half of the tortilla, layer a generous amount of shredded Mexican cheese blend. Spoon a portion of the chicken filling over the cheese, then sprinkle a little more cheese on top. Carefully fold the tortilla in half, pressing down gently to seal the edges.

In a large skillet, heat 1 tablespoon of butter over medium heat until melted and begins to bubble slightly.

Place the folded tortilla in the hot skillet and cook for 2-3 minutes on one side. Check for a golden-brown color before flipping it. Once the bottom is crispy, carefully flip the quesadilla and cook the other side for another 2-3 minutes.

After both sides are golden and crispy, remove the quesadilla from the skillet and let it rest on a cutting board for a minute. Cut the quesadilla into wedges and serve hot, perhaps with extra ranch dressing or salsa for dipping.