Let the rack of lamb come to room tempurature for 15 minutes. While the rack of lamb is resting, preheat the oven to 425 degrees. Line a baking sheet with parchment paper.
Make the sauce. Combine the dijon mustard, maple syrup, and finely chopped rosemary in a small dish. Place the rack of lamb on the baking sheet. Use a paper towel to dry the lamb. Using a culinary brush, coat the rack of lamb with the sauce.
Coat the lamb with the pecans. Finley chop the pecans so they will stick easier to the rack of lamb. Using your hands, pat the pecans onto the lamb.
Cook the lamb. Place the baking sheet in the oven in the middle rack. Place a tinfoil tent over the rack of lamn loosely to prevent the pecans from burning. Allow the lamb to cook for 30 minutes, until the meat thermometer reads 135 degrees (medium rare).
Serve. Allow the lamb to rest for 10 minutes before cutting to allow the juices to redistribute. Cut between the bones and serve with your favorite roasted vegetables. Mangia!