Combine chiles, cheese, and cilantro in a medium bowl and toss to evenly mix.
Heat a seasoned cast iron skillet or a large frying pan over medium heat. Add 1 quesadilla and cook until soft, about 15 to 30 seconds.
Add 1/2 cup cheese mixture on one half, fold in half, and cook until cheese is melted and tortilla is golden brown, about 1 to 2 minutes. Repeat for remaining quesadillas. Cut each quesadilla into 4 wedges and serve with tomatillo salsa.