1. Heat the oven to 450°. Coat the pork with the 1 tablespoon of mustard and the 1 1/2 teaspoons of oil. Sprinkle it with 1/2 teaspoon of the salt and 1/4 teaspoon of the black pepper. Put the pork on a baking sheet and roast until just done, 20 to 25 minutes. Transfer the pork to a carving board and leave to rest in a warm spot for 5 minutes.
2. Meanwhile, put the onion in a small bowl of cold water and let stand for 10 minutes. Drain the onion, rinse, and then pat dry with paper towels.
3. In a large glass or stainless-steel bowl, whisk together the lime juice, the 1 teaspoon mustard, and the remaining 1/4 teaspoon each salt and black pepper. Add the 1/4 cup oil slowly, whisking. While the pork is still warm, cut it into bite-size pieces. Add the pork, onion, red-pepper flakes, and cilantro to the vinaigrette and toss.
Variations: Leftover pork or beef would not only taste great, it would actually be more traditional. The meat should really be cooked through, though, not rare.
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