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.
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.
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.
Wine Recommendation: Merlot's soft, supple texture and ripe, juicy plum and chocolate flavors have made it a current favorite among red wines. Savor a well-made version from California's Napa Valley here, and you'll know what all the fuss is about.