Step 1

To a food processor, add the garlic cloves, the leaves from the sprigs of rosemary, and the salt. Pulse to finely chop these ingredients before adding in the mustard and balsamic vinegar. Blend again to mix everything together. If you're not using a food processor, you can finely mince the cloves and rosemary with a sharp knife, then combine everything in a small bowl. Taste test to add more salt, if needed.

Step 2

Place the rack of lamb on a meat board, with the rib bones curving down so that the fatty layer on the lamb is facing on top. Spread the rosemary-mustard marinade all over the top of the lamb meat. Let the lamb rest for at least 30 minutes, or up to 1 hour. You can also place the lamb covered in marinade in the fridge overnight, but be sure to let it sit on the counter for a bit the next day before putting it in the oven. (Best to do this before preheating, or just as you are about to preheat, the oven).

Step 3

Preheat the oven to 450°F. Transfer the rack of lamb to a baking sheet lined with foil. Then place the lamb in the middle rack of the oven to cook for approximately 20 minutes, or until the lamb reaches 140°F to 145°F when measured with a food thermometer. You'll then cover the lamb with foil and let it rest for about 5 to 10 more minutes. This will produce a medium lamb. If you want a more pink or rare lamb, check on the lamb before 20 minutes (about 15 to 18 minutes in) and check for a temperature of 130°F. Then pull the lamb out of the oven, cover with foil and let it rest for 5 to 10 minutes. If you overcook your lamb in the oven, be sure not to let the lamb rest for more than a minute or so outside the oven. Covering the lamb with foil and letting it rest on the baking sheet outside the oven allows for the lamb to continue cooking, so if you've already overcooked the lamb, you don't want to cook it any further.

Step 4

Separate the lamb pieces by using a sharp knife to cut in between the bones. Serve with your choice of sides.