Heat oven to 400° F. Prick the potatoes all over with a fork and bake directly on an oven rack until tender, 45 to 55 minutes.
Meanwhile, in a shallow dish, combine the garlic, rosemary, and oil. Add the steaks and turn to coat. Let marinate, turning once, for at least 30 minutes (and up to 1 day in the refrigerator).
Heat a large skillet over medium heat. Remove the steaks from the marinade (reserve the marinade) and season with 1/2 teaspoon each salt and pepper. Cook to desired doneness, 5 to 7 minutes per side for medium-rare. Let rest at least 5 minutes before slicing.
Wipe out the skillet and return to medium heat. Add the reserved marinade and cook, stirring occasionally, until the garlic is golden, 2 to 3 minutes. Spoon over the sliced steak and serve with the potatoes and sour cream.