Preheat oven. Cut beets into small wedges about 1-inch thick (either halved or quartered), then place in a large roasting dish. Drizzle with olive oil, Koser salt, and black pepper, then cover with foil and roast.
Let cool slightly, then use a kitchen towel or paper towel to gently rub off the skin. Set aside. Meanwhile, make the horseradish cream.
Combine sour cream, prepared horseradish (depending on how spicy you want the sauce), and olive oil in a medium bowl, then whisk to combine. Taste, adjust seasoning, then set aside.
Layer the cooked beets on top. Sprinkle with arugula microgreens, walnut pieces, and a sprinkling of flaky sea salt. Finish with a generous drizzle of high quality extra virgin olive oil. Enjoy immediately or at room temperature!