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.
Heat a grill pan or gas grill over medium to medium-high (or start a charcoal fire and let it burn until the coals are medium hot and covered with white ash). Lightly brush or spray the onion slices with oil, sprinkle with salt and lay on the grill to cook until golden, but still crunchy, about 4 minutes. Move to a cool part of the grill to keep warm.
Brush the meat with oil and lay on the hottest part of the grill. When the meat releases itself from the grates, turn it over and finish 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 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 or hot sauce for making soft tacos.