Steak tacos are Mexico’s taquería crowd pleaser. Who doesn’t enjoy a chamois-soft, warm corn tortilla wrapped around slices of spicy, smoky, chipotle-smeared seared beef and caramelized onions?
Turn the oven on to its lowest setting. Open the can of chipotles, and scrape it into a food processor or blender. Process until smooth. Use a brush to smear a light chipotle patina on the steak (go a little heavier on flank than skirt steak). You’ll have chipotle marinade leftover; cover and refrigerate for up to several weeks.
In a large (12-inch) skillet—the heavier the better—heat 2 tablespoons of the oil over medium-high. Add the onion and cook, stirring frequently, until golden, but still crunchy, about 4 minutes. Scoop into a heat-proof serving bowl, leaving as much oil as possible in the skillet, and slide into the oven.
Return the skillet to medium-high heat (a little lower for flank steak). Add the remaining 1 tablespoon of the oil. When hot, lay in the steak(s). Brown on both sides, cooking as done as you like: about 3 minutes per side for medium skirt steak, about 5 minutes per side for 1 ½-inch-thick, medium-rare flank steak.
The meat will be juiciest if you let it rest in the oven for 5 to 10 minutes on a wire rack over a plate, but you can cut and serve it right away. Cut the skirt steak into roughly 3-inch lengths, then cut each piece across the grain into ¼-inch strips. Cut the flank steak in ¼-inch slices straight down across the grain; on the thick end, you may want to cut the slices in half lengthwise into thinner strips.
Toss the meat with the onions. Taste and season with salt, usually about 1 teaspoon. Serve with the warm tortillas and salsa for making soft tacos.