Notes: This tangle of sautéed onions and roasted chilies--rajas--has the simple, direct flavors often found in Mexico's tacos. It's from Claire Archibald, chef and co-owner with her sister, Shawna, of Portland's Café Azul. Tuck the cheese into the warm tortillas and top with the rajas.
1. Rinse and dry chilies. Arrange in a single layer in a 10- by 15-inch pan. Broil 4 to 5 inches from heat, turning to char chilies on all sides, about 10 minutes total. Let stand until cool to touch, 8 to 10 minutes. Pull off and discard chili skins, stems, and seeds. Cut chilies lengthwise into thin strips.
2. Meanwhile, peel onions and cut lengthwise into slivers. In a 12-inch frying pan or a 5- to 6-quart pan over medium-high heat, frequently stir onions, oil, and 1 teaspoon salt until onions are lightly browned, 7 to 8 minutes. Add chilies and stir occasionally to blend flavors, about 5 minutes. Season to taste with salt.
3. Thinly slice cheese. See notes for assembly.