Preheat oven to 350 degrees and line a large, rimmed baking sheet with parchment paper or a silicone baking mat.Prepare the taco seasoning by combining all ingredients in a small bowl and stir to combine. Set aside.
Make the Pico de Gallo: Finely, dice the cherry tomatoes, red onion, and serrano peppers. Chop the cilantro (if using). Mince the garlic and juice the lime.
Then combine all ingredients, including the salt and pepper, in a medium glass or other non-reactive bowl and stir to combine. Set aside.
Making the Chicken Quesadillas: Add the shredded rotisserie chicken to a large bowl and set aside.
Combine the sour cream and taco seasoning in a smaller bowl and stir to combine.
Add one-half cup of the seasoned sour cream to the shredded chicken and season with additional salt and black pepper, to taste. Toss to combine and set aside. Be sure to reserve the remaining seasoned sour cream for serving.
Heat a large, non-stick skillet over medium heat. Add a chunk of butter and spread over the bottom of the skillet with a spatula as it melts. Add a tortilla to the hot skillet and toast on one side for about 1 minute, or just until golden brown, as shown above.
Transfer the toasted tortilla to the prepared baking sheet, toasted side down. Carefully wipe out the skillet with a clean, damp cloth, then add another chunk of butter. Once the bottom of the skillet is coated, add a second tortilla.
While the second tortilla is toasting, add one-sixth of the seasoned chicken meat and one-quarter cup of the shredded cheese to one half of the non-toasted side of the first tortilla. If using, add the chopped cilantro at this stage, as well.
Fold the tortilla over, as shown, and press down with a spatula to set. Set aside.
Repeat Steps #7 - #10 until all tortillas are toasted, filled, and arranged on the prepared baking sheet. Place the large, rimmed baking sheet in the pre-heated oven for 8-10 minutes, or just until the cheese is melted and the quesadilla is heated through.
Remove from oven and cool slightly before cutting each quesadilla into thirds. Sprinkle with some fresh cilantro and serve immediately with lime wedges, diced avocado (or guacamole), the Pico de Gallo, and the remaining seasoned sour cream on the side.