1. Cut pork crosswise into thin strips. (If using tenderloin, first cut into 1/2-in.-thick slices. It's easiest to do this if the meat is partially frozen.) Sprinkle the pork strips with the pepper seasoning, tossing to coat.
2. In a large, heavy skillet (or wok), heat only 2 tbs. of the oil over high heat. Add the pork and cook, undisturbed, until it begins to blacken a bit on the bottom, 1-2 min. Stir and cook 1 more min. until the meat is no longer pink inside. Using a slotted spoon, remove the strips to a plate.
3. Reduce the heat to med-hi and add the remaining tbs. of oil to the pan. Add the garlic and half the watercress and cook, turning with tongs, until the watercress begins to wilt; then add the remaining watercress. Cook, turning, about one min. Return the pork and any accumulated juices to the pan, add the broth and soy sauce, and simmer for 1 min. to blend the flavors. Sprinkle with the peanuts before serving.
