Preheat oven to 400 degrees F.
Roast the pork, fat side down, for about 30 minutes. Turn over and cook until an instant- read thermometer registers 150 degrees F when inserted into the thickest part of the roast, about 30 minutes longer. Transfer the roast to a carving board, tent with aluminum foil and set aside to rest.
Pour the beef broth into the roasting pan and, with a wooden spoon, scrape up the brown bits that are stuck to the bottom of the pan. If the brown bits are stubborn, set the roasting pan over low heat and continue stirring. Strain the pan juices and set aside.
Heat the oil in a large skillet until smoky and add the vegetables. Cook, stirring, until tender-crisp. Sprinkle with flour and stir a few minutes to remove any raw taste from the flour. Stir in the strained pan juices, adjust the heat so the liquid is simmering, and season, to taste, with salt and pepper.
Cut the roast into 1/2-inch slices. Spoon some of the onion-pepper sauce over each slice and sprinkle a little of the chopped cilantro over each.
Yield: about 1 cup