Broil 3 fresh poblano chiles, turning to char and blister on all sides. Let cool. Discard stems and seeds; cut chiles into thin strips.
Cook 1 sliced white onion in 1 tbsp. butter over medium heat until softened, 4 to 5 minutes. Stir in chile strips and top with jack cheese slices.
Cover and cook over low heat until cheese melts, about 2 minutes. Scoop chile mixture into 6 small warm corn or flour tortillas.
Time: 20 minutes