Peel and dice the potatoes into 1/2-inch cubes. Place the potatoes in a medium pot with a generous pinch of salt and enough water to cover. Bring to a boil, then reduce to simmer and cook for 10-12 minutes, until fork-tender. Drain.

Meanwhile, thinly slice the scallions, keeping the greens and whites separate. Peel and mince or grate 1 teaspoon ginger.

Make the soy glaze: in a small bowl, combine the soy sauce, stock concentrate, 1 teaspoon ginger, 1 tablespoon sugar, and 1/4 cup water.

Heat a drizzle of oil in a large pan over medium heat. Season the steak on all sides with salt and pepper. Add the steak to the pan and cook for 4-7 minutes per side, or until cooked to desired doneness. Set aside to rest for 5 minutes. Wipe out any black bits in the pan with a paper towel.

While the steak cooks, heat 1 tablespoon butter and the scallion whites in the same pot you cooked the potatoes in over medium heat. Cook for 30 seconds, until melted and fragrant. Add the drained potatoes, sour cream, and the wasabi (to taste, start with a tiny bit and go up from there!) Mash thoroughly with a fork or potato masher until smooth, adding a splash of water if necessary. Taste and season with salt and pepper.

In the same pan you cooked the steak in, heat a drizzle of oil in a large pan over medium heat. Add the sesame seeds and toast, tossing constantly, for 30 seconds Add the spinach and a splash of water to the pan and cook, tossing, until wilted, for 1-2 minutes. Season with salt and pepper, then set aside, covered, to keep warm.

Add the soy glaze to the same pan you cooked the spinach in. Bring the mixture to a simmer over medium heat and reduce until slightly thickened, for 1-2 minutes.

Thinly slice the steak against the grain and serve alongside the spinach and wasabi mashed potatoes. Drizzle the soy glaze over the steak and garnish with the scallion greens. Enjoy!