Season the roast well with salt and pepper. Heat 1 tablespoon olive oil in a large ovenproof (preferably cast iron) sauté pan over medium-high heat, and add 1 tablespoon of the oil. When hot, sear the beef on all sides, until nicely browned, about 8 minutes in total. Transfer the beef to a plate for a few minutes, and pour off most of the remaining fat in the pan.
Meanwhile, in small bowl, whisk together the remaining 2 tablespoons olive oil, the minced garlic, Dijon mustard, balsamic vinegar, ½ teaspoon kosher salt, and pepper to taste.
Brush or smear the mixture all over the roast, and return the beef to the skillet fat side down. Roast in the oven until an instant read thermometer registers 125°F for medium-rare (25 to 30 minutes), or 130°F for medium (30 to 35 minutes; the temperature will continue to climb for a bit after it is removed from the oven.)
Let the roast rest on a cutting board for 10 minutes for it to finish cooking and for the juices to reabsorb into the meat.
While the beef is cooking, make the Horseradish Sauce: in a small bowl blend together the sour cream, horseradish, lemon juice and salt. Thinly slice the meat and serve with the sauce, and the starch of your choice (ideally mashed potatoes!)