Microwave the vinegar, sugar, 1 teaspoon of the salt, and ¼ teaspoon of the pepper in a microwave-safe bowl until hot, about 30 seconds. Stir to dissolve the sugar. Add the cabbage and half the scallions; toss to coat.
Sprinkle the steak with 1 teaspoon of the salt and the remaining pepper. Cut the plantain in half crosswise, then lengthwise into 4 long slices. Sprinkle with the ground chipotle and the remaining salt.
Heat the oil in a large cast-iron skillet over medium-high and cook the steak, 4 to 5 minutes per side for medium-rare. Transfer to a cutting board; let rest 5 minutes before slicing against the grain. Meanwhile, add the plantains to the skillet and cook until golden, 2 minutes per side.
Divide the steak, plantains, sour cream, avocado, and the remaining scallions among the tortillas. Serve with the slaw.
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