Prep the lamb: Pat the lamb dry. If using a bone-in leg, trim any very thick exterior fat, leaving a thin layer for flavor. For boneless, tie it with kitchen twine so it roasts evenly.

Season early: Sprinkle the lamb generously with kosher salt and pepper on all sides. For best results, do this 12–24 hours ahead and refrigerate uncovered. If you’re short on time, 30 minutes is still helpful.

Make the rosemary-lemon paste: In a small bowl, combine 3 tablespoons olive oil, 2 tablespoons chopped rosemary, the zest of 2 lemons, the juice of 1 lemon, 6 minced garlic cloves, 1 teaspoon Dijon mustard (optional), and a pinch of red pepper flakes. Stir into a thick paste.

Rub and rest: Bring the lamb to room temperature for 45–60 minutes before roasting. Rub the paste all over, massaging it into any crevices. Tuck a few whole rosemary sprigs under the roast for aroma.

Preheat and pan setup: Heat the oven to 450°F (230°C). Set the lamb on a rack in a roasting pan. If you’d like a built-in side, scatter chunked potatoes and carrots underneath, tossed with olive oil, salt, and pepper to catch the drippings.

Roast hot, then low: Roast at 450°F for 15–20 minutes to brown. Reduce heat to 325°F (165°C) and continue roasting until the thickest part reaches your preferred doneness: 125–130°F for medium-rare, 135°F for medium. Expect roughly 12–15 minutes per pound after the initial sear, but start checking early.

Rest the roast: Transfer lamb to a cutting board and tent loosely with foil. Rest 20–25 minutes. The temperature will rise by a few degrees, and the juices will redistribute.

Make a quick pan sauce: Place the roasting pan over medium heat. Spoon off excess fat, leaving about 1 tablespoon. Add 1/2 cup white wine or broth and scrape up brown bits. Simmer 2–3 minutes, then swirl in 1–2 tablespoons butter and a squeeze of lemon. Season to taste.

Slice and serve: Slice against the grain into 1/2-inch pieces. Serve with roasted vegetables and spoon the pan sauce over the top. Garnish with a few fresh rosemary needles and lemon zest for brightness.