Preheat your oven to 350°F/180°C/Gas 4. Line a large, rimmed baking sheet with parchment paper or a nonstick baking mat and set it aside.
Place the chili powder, ground cumin, ground coriander, garlic powder, smoked paprika, chipotle powder, black pepper, and sea salt into a small bowl. Stir to combine and set the bowl aside.
Place the tomatoes, red onion, cilantro, lime juice, serrano peppers, and garlic into a medium glass or non-reactive bowl. Season with salt and pepper to taste and stir to combine.
Place the sour cream and taco seasoning into a small bowl and stir to combine. Place one-half cup of the seasoned sour cream into a large bowl with the shredded rotisserie chicken. Season with salt and pepper to taste, if desired. Toss to combine and set the seasoned chicken aside. Reserve the remaining seasoned sour cream for serving.
Heat a large non-stick pan or skillet over medium heat. Add some butter and spread it over the pan with a spatula as it melts. Add a tortilla to the hot pan and toast on one side for about 1 minute, or until just golden brown. 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 to one-half of the non-toasted side of the first tortilla. Top with one-quarter cup of shredded cheese, and some chopped cilantro if using. Fold the other half of the tortilla over and press down with a spatula to set it. Set the baking sheet aside.
Repeat steps 5 and 6 until all the tortillas are toasted, filled, and arranged on the prepared baking sheet. Place the full baking sheet in the center of the preheated oven for 8-10 minutes, or until the cheese is melted and the quesadillas are heated through.
Remove the quesadillas from the oven and leave them to cool slightly before cutting each one into thirds.
To serve, arrange the cut quesadilla onto a plate, sprinkle with some fresh cilantro, lime wedges, diced avocado (or guacamole), the Pico de Gallo, and the remaining seasoned sour cream on the side. Enjoy!