1 tbsp finely chopped rosemary, plus springs for garnish
1 tbsp minced garlic
2 tbsp olive oil
Preheat oven to 450°F. Season the roast all over with 1 tsp salt and place in a shallow roasting pan, rib-side down. Roast 20 min.
Meanwhile, in a medium bowl, stir together the mustard, peppercorns, rosemary, garlic, and remaining ½ tsp salt. Stir in the oil until well combined.
Reduce oven temperature to 325°F. Brush mustard mixture all over top and sides of roast. Place in oven and cook 1½–2 hours (about 10 min. per lb for medium-rare), until thermometer inserted into thickest part of roast reaches 120°F for medium-rare or 130°F for medium, rotating roasting pan 2–3 times.
Transfer roast to a carving board. Let rest 20 min., tenting loosely with foil.
Thinly slice roast and arrange on a large serving platter.