In a small bowl, mix together chili powder, salt, and garlic powder. Place the cooked chicken in a separate bowl and sprinkle with half of the spice mixture. Stir to coat and set aside.
In a large skillet over medium heat, heat olive oil until shimmering. Add onion and bell peppers. Sprinkle with the remaining spice mixture and sauté for 6–8 minutes until onions are translucent and peppers are crisp-tender. If using spinach, stir it in at the end until wilted. Add hot sauce if desired.
Transfer the sautéed vegetables to the bowl with the seasoned chicken and stir to combine.
Lay out a tortilla. Sprinkle with 2 ½ tablespoons pepper Jack and 1 ½ tablespoons cheddar. Spread ¼ of the chicken-vegetable mixture over the cheese, then top with another 2 ½ tablespoons pepper Jack and 1 ½ tablespoons cheddar. Fold the tortilla in half.
Wipe out the skillet and heat over medium-low heat. Add a small amount of oil or butter if desired. Place one or two quesadillas in the pan (folded edge down). Press lightly with a spatula. Cook for about 4 minutes per side, until golden brown and cheese is melted.
Transfer cooked quesadillas to a cutting board. Let rest for 5 minutes, then slice into wedges. Serve hot with salsa, guacamole, and sour cream.
