Heat grill to medium-high.
Place the potatoes in a large pot. Add enough cold water to cover. Bring to a boil. Reduce heat and simmer until tender, 15 to 20 minutes.
Meanwhile, season the steak with 1/2 teaspoon of the salt and 1/4 teaspoon of the black pepper. Grill until cooked to the 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, vinegar, and the remaining salt and black pepper and pulse. Drain the potatoes, return them to the pot, add 1 cup of the romesco sauce, and toss. Divide the potatoes, steak, and watercress among individual plates. Spoon the remaining sauce over the steak.
Tip: You can also spoon this rich, earthy romesco sauce over grilled chicken, broiled fish, roast vegetables, baked potatoes, or fried eggs.