Toss together beet, carrot, scallions, cilantro, cider vinegar, and salt in a bowl. Let rest at least 30 minutes. Stir together sour cream, horseradish, and lime juice.
Season steak with salt and pepper. Heat oil in a skillet over medium-high heat. Cook steak for 4 minutes per side, or done to liking. Let rest 5 minutes and then thinly slice.
Place steak slices on tortillas and top with slaw and sour cream mixture.