Heat the oven on its lowest setting and arrange a rack in the middle. Place the chipotles and adobo sauce in a food processor fitted with a blade attachment or a blender and process until smooth. Using a brush, lightly coat the steak with the chipotle purée (go a little heavier on the flank than the skirt steak; you’ll have chipotle marinade left over—cover and refrigerate for up to several weeks). Cut the steak crosswise into 6-inch pieces.
Heat 2 tablespoons of the oil in a large frying pan over medium-high heat until shimmering. Add the onions and cook, stirring frequently, until golden but still crunchy, about 4 to 5 minutes. Transfer to a large heatproof bowl or dish, leaving as much oil as possible in the skillet, and place in the oven to keep warm.
Return the frying pan to medium-high heat (or medium heat for flank steak), add the remaining 1 tablespoon oil, and heat until shimmering. Add the steak and brown on both sides, cooking until it reaches your desired doneness (about 3 minutes per side for medium skirt steak, about 5 minutes per side for medium rare for a 1-1/2-inch-thick flank steak). Transfer the steak to a wire rack set over a plate to rest, about 5 to 10 minutes.
Slice the skirt steak into roughly 3-inch lengths, then cut each piece against the grain into 1/4-inch strips. For flank steak, cut it straight down against the grain into 1/4-inch slices; on the thick end, you may want to cut the slices lengthwise in half into thinner strips.
Place the meat in the bowl with the onions, add the measured salt, and toss to combine. Taste and season with more salt as needed. Serve with the warm tortillas and salsa or hot sauce.