1. Mix the lemon juice, zest, fennel seeds, garlic and rosemary with about 4 tbsp olive oil. Rub over the lamb and place in a zip-loc bag into the fridge overnight.
2. Preheat the oven to 190C /Gas mark 5. Bring the lamb to room temperature, which will take about half an hour, strain it from the marinade pat it dry and put it into the roasting tin.
3. Place the tin on the hob and brown the lamb on all sides. Then place a rack in the roasting tin and put the lamb on top, skin side up. Season with salt and pepper and roast the lamb for 40-45 minutes for medium rare, basting it from time to time. Cook for a further 10 minutes if you don't like your lamb too pink.
4. Allow it to rest for ten minutes in a warm place.
5. Meanwhile, to make the dressing, whisk all ingredients together to emulsify them. Then transfer the lamb onto a serving platter with the roasted pumpkin and garlic. Spoon over the lemon dressing and serve.