Boil the potatoes in salted water until nice and tender, and drain. Then add the butter, allow it to melt, and mash until mostly smooth.
Mix in the wasabi and sour cream, and add salt to taste.
Start a small skillet over medium heat and melt the butter, then add the scallions, garlic, and ginger and cook for around a minute, until nicely fragrant.
Add the flour and stir it around. Cook for a minute or two more, allowing the flour to begin to brown.
Next, add the broth and soy sauce, stirring well to break up any lumps. Cook until the gravy thickens to your desired consistency, remembering that it will further thicken as it cools.
Serve the mash and gravy alongside grilled steak, teriyaki chicken, or tofu katsu with a side of steamed broccoli for a perfect meal.
