The classic pairing of lamb and mint gets another layer of flavor with the addition of spring onions or leeks.
Preheat the oven to 475 degrees. Place the lamb in a roasting pan fitted with a rack.
Put the garlic, fennel seeds, salt, and pepper in the bowl of a food processor. Pulse until coarsely ground, stopping to scrape down the sides of the bowl several times. Transfer the mixture to a small bowl and stir in 1 tablespoon of the oil. Rub the mixture over lamb.
Roast the lamb for 10 minutes. Reduce the oven temperature to 300 degrees and continue roasting until lamb reaches the desired level of doneness, about 1 ½ hours for medium. Transfer the lamb to a cutting board and let rest for 15 minutes.
Meanwhile, heat the remaining oil in a large skillet over medium-high heat. Add the onions and fennel and cook for 2 minutes. Add the wine, cover, and continue to cook until the vegetables are very soft, about 10 more minutes. Season with salt, to taste, and stir in the mint leaves. Carve the lamb and serve the slices with the spring onion mixture.