Preheat broiler. Put chiles on a baking sheet and broil about 3 in. from heat, turning as needed to char and blister on all sides, about 8 minutes total.
Let cool. Discard stems and seeds.
Cut chiles into thin strips.
Meanwhile, in a 6- to 8-in. frying pan over medium-high heat, cook onion in butter, stirring often, until softened, 4 to 5 minutes.
Stir in chile strips and top with a layer of cheese slices. Cover and cook over low heat until cheese melts, about 2 minutes. Scoop hot chile mixture into tortillas.
