How to Make It
Preheat the oven to 450° and preheat a cast-iron grill pan. In a bowl, combine the wasabi and water. Stir in the horseradish and soy sauce. Rub the steak with 1 teaspoon of the oil and season with salt and pepper. Grill the steak over high heat until lightly charred, 5 minutes. Flip the steak and spread the wasabi over the charred side.
Transfer the pan to the oven and roast the steak for 10 minutes, until an instant-read thermometer inserted in the thickest part registers 135° for medium-rare; transfer to a cutting board and let rest for 10 minutes.
Meanwhile, in a saucepan of boiling water, cook the potatoes for 15 minutes. Drain and let cool, then peel. Wipe out the saucepan. Add the remaining oil and the potatoes and cook over moderate heat, stirring occasionally, until golden, 5 minutes. Combine the miso and mirin; add to the potatoes and cook, stirring, until glazed, about 2 minutes.
Thinly slice the steak across the grain and serve with the potatoes and watercress.