Cut chiles in half lengthwise; discard seeds and membranes.
Place chile halves, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil 8 minutes or until blackened.
Place in a zip-top plastic bag; seal.
Let stand 15 minutes. Peel and chop. Reduce oven temperature to 20
Heat oil in a large nonstick skillet over medium-high heat.
Add mushrooms; saut 2 minutes.
Add corn, onions, and pepper; saut 2 minutes.
Place mixture in a bowl; stir in chopped chiles. Wipe pan clean.
Place about 1/4 cup mushroom mixture and 1/4 cup cheese over half of 1 tortilla. Repeat procedure with remaining 1 1/4 cups mushroom mixture, remaining 3/4 cup cheese, and remaining 3 tortillas.
Heat pan over medium heat. Coat pan with cooking spray.
Place 1 tortilla in pan; cook 2 minutes or until cheese melts and bottom is golden. Fold tortilla in half; place on a baking sheet.
Place in 200 oven to keep warm. Repeat procedure with remaining tortillas.
Cut each quesadilla into wedges; serve with salsa.