1. Place chilies in a broiler pan and broil about 3 inches from heat, turning as needed to char and blister on all sides, about 8 minutes total. Let cool. Pull off and discard stems and seeds. Cut chilies into thin strips.
2. Meanwhile, in a 6- to 8-inch frying pan over medium-high heat, stir butter and onion often until onion is limp, 4 to 5 minutes.
3. Mix chilies with onions and lay cheese slices on the vegetables. Cover and cook over low heat until cheese melts, about 2 minutes.
4. Scoop hot cheese mixture into tortillas. Fold to enclose filling, and eat.
Nutritional analysis per appetizer serving.