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1
Heat the oven on its lowest setting and arrange a rack in the middle.
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2
Place the chipotles and adobo sauce in a food processor fitted with a blade attachment or a blender and process until smooth.
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3
Using a brush, lightly coat the steak with the chipotle puree (go a little heavier on the flank than the skirt steak; youll have chipotle marinade left overcover and refrigerate for up to several weeks).
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4
Cut the steak crosswise into 6-inch pieces.
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5
Heat 2 tablespoons of the oil in a large frying pan over medium-high heat until shimmering.
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6
Add the onions and cook, stirring frequently, until golden but still crunchy, about 4 to 5 minutes.
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7
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.
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8
Return the frying pan to medium-high heat (or medium heat for flank steak), add the remaining 1 tablespoon oil, and heat until shimmering.
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9
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).
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10
Transfer the steak to a wire rack set over a plate to rest, about 5 to 10 minutes.
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11
Slice the skirt steak into roughly 3-inch lengths, then cut each piece against the grain into 1/4-inch strips.
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12
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.
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13
Place the meat in the bowl with the onions, add the measured salt, and toss to combine.
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14
Taste and season with more salt as needed.
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15
Serve with the warm tortillas and salsa or hot sauce.
