How to Make It
Heat grill to medium-high. Place the potatoes in a large pot. Add cold water to cover. Bring to a boil. Reduce heat and simmer until tender, 15 to 20 minutes.
Meanwhile, season the steak with ½ teaspoon of the salt and ¼ teaspoon of the pepper. Grill until cooked to desired doneness, 3 to 5 minutes per side for medium-rare. Let rest for 10 minutes before slicing.
Meanwhile, in a food processor, pulse the almonds. Add the garlic, Parmesan, red peppers, oil, and vinegar and the remaining salt and pepper. Pulse.
Drain the potatoes, return them to the pot, add 1 cup of the sauce, and toss.
Divide the potatoes, steak, and watercress among individual plates. Spoon the remaining sauce over the steak.