Remove the lamb from the fridge and allow to come to room temperature while you prepare the stuffing.
Heat half the oil in a large frying pan, add the spices and cook for 1 min, then scoop out into a large bowl. Add the gluten free breadcrumbs, herbs, feta, lemon and tahini.
Season and mix everything together. Unroll the lamb and season well on both sides.
Pack the stuffing down the centre of the lamb, pressing to help it stick, then re-roll and tie up with kitchen string.
Flip the lamb over so it is seam-side down, and pack any stuffing that has fallen out back in. Heat oven to 200C/180C fan/gas 6.
Now assemble the gratin. Slice the pumpkin in half then deseed. Peel both pieces with either a sturdy peeler or a sharp knife.
Slice very thinly into half moons. Whisk the tahini, lactose free cream and stock in a jug with some seasoning.
Arrange the sliced pumpkin in a shallow ovenproof baking dish, approximately 22 x 30cm, seasoning well between the layers. Finish with a layer of neatly lined-up half moon slices. Pour over the tahini stock.
Place the lamb on top of the gratin, seam-side down, and rub with the remaining oil. Place in the centre of the oven and roast for 1 hr 20 mins for slightly pink lamb.
Lift the lamb off the gratin and transfer to a plate, covered with foil to keep warm, and leave to rest. 4
While the lamb rests, increase oven to 220C/200C fan/gas 7.
Spoon any excess fat off the top of the gratin, then return to the oven for 20 mins until golden and crisp. Place the lamb back on top of the gratin to serve, scattered with mint leaves