Trim all fat off of the leg of lamb.

Butterfly the leg of lamb.

Place garlic cloves in a food processor and finely chop.

Add dried rosemary, dark brown sugar, kosher salt, fresh ground black pepper, and mustard to the food processor.

Blend all ingredients until a paste forms.

Spread the rosemary-mustard paste evenly on the outside of the lamb.

Cover the lamb and refrigerate for at least 4 hours, or preferably overnight.

Remove the lamb from the refrigerator and let it sit at room temperature for at least 30 minutes.

Insert a digital thermometer into the thickest part of the lamb.

Set the thermometer to 155°F (for medium-rare) or your desired internal temperature.

Preheat the oven to 400°F.

Place the lamb in the preheated oven.

After 15 minutes, reduce the oven temperature to 350°F.

Continue cooking the lamb until the thermometer reaches the set temperature.

Estimate cooking time as 15 minutes plus 20 minutes per pound.

Once the lamb is cooked, remove it from the oven and let it rest for 15-20 minutes.

Carve the lamb and serve.