How to Make It
Heat the oven to 450°. Coat the pork with the 1 tablespoon of mustard and the 11/2 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.
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.
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.
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.