Position the racks in the upper and lower thirds of the oven; preheat to 450°. Place the roast on a foil-lined baking sheet. Rub with the oil; season generously with the salt and pepper. Arrange half of the thyme sprigs under the roast and tuck the remaining sprigs under the twine all over the roast.
Roast on the lower rack until an instant-read thermometer inserted into the center registers 125° for medium-rare, 40 to 45 minutes. Let rest for 10 minutes.
Meanwhile, in a small bowl, whisk the sour cream, horseradish cream, heavy cream, oregano, and lemon juice; season. Serve with the roast.
This recipe is part of our 1-hour holiday dinner menu. Get the rest of the recipes here.