Remove meat from refrigerator and set out at room temperature for 1 hour. Preheat oven to 450°F.

Set meat, bone side down, in a roasting pan and season it lightly with salt. Roast meat in the lower third of the oven for 20 minutes. Remove meat from oven and reduce temperature to 350°F.

In a small bowl, mix mustard with garlic, thyme, pepper and 2 teaspoons kosher salt. Whisk in olive oil. Brush mustard coating all over top and sides of meat and roast for about 1 1/2 hours longer, rotating roasting pan 2 or 3 times for even browning. The meat is done when an instant-read thermometer inserted in the center of the roast at the thickest part registers 120°F (for medium rare). If you prefer meat cooked to medium, cook roast to an internal temperature of 130°F. 

Transfer roast to a carving board, cover loosely with foil and let rest for 20 to 30 minutes. Set roast on its side and run a long, sharp knife between bones and meat; remove bones and set them aside. Turn roast right side up. Carve roast 1/4 to 1/2 inch thick and transfer slices to warmed plates. Pour any carving juices over meat and serve immediately.