How to Make It
Preheat the oven to 450°. In a small bowl, mix the mustard with the garlic, thyme, pepper and 2 teaspoons of kosher salt. Whisk in the olive oil.
Set the meat, bone side down, in a roasting pan and season it lightly with salt. Roast the meat in the lower third of the oven for 20 minutes.
Remove the meat from the oven and reduce the temperature to 350°. Brush the mustard coating all over the top and sides of the meat and roast for about 1 1/2 hours longer, rotating the 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° (for medium rare). Transfer the roast to a carving board, cover it loosely with foil and let rest for 20 to 30 minutes.
Set the roast on its side and run a long, sharp knife between the bones and meat; remove the bones and set them aside. Turn the roast right side up. Carve the roast 1/4 to 1/2 inch thick and transfer the slices to warmed plates. Pour any carving juices over the meat and serve at once. For bone-gnawing carnivores, cut down between the rib bones and pass them on a plate.