Remove the turkey from the fridge and allow it to come up to room temperature. Put a shelf on the lowest level in your oven (so you have enough room for your turkey to go on this and then for two shelves above the bird). preheat the oven to gas 4, 180°C, fan 160°C. To make the butter, combine the softened butter with the allspice, the garlic, the parsley and the lemon zest. Take your turkey and stuff half the butter under the skin, being careful not to tear it, then rub the remaining butter all over the turkey itself. Halve the zested lemons and put them inside the cavity. Season well. Put the turkey on the rack of a large roasting tin and pour the stock into the bottom. Cover with foil and cook for 2 hours 30 minutes for a 5·.5kg (11lb) bird. Put the unpeeled shallots on a separate baking tray, drizzle with olive oil, season, and roast for 1 hour. Remove from the oven, setting aside 6 shallots to garnish the turkey later, then peel and roughly chop the remaining shallots for the gravy. Set aside until needed. After 2 hours 30 minutes, remove the foil from the bird, then increase the heat to gas 7, 220°C, fan 200°C and cook for a further 30 minutes. To check if the turkey is cooked, insert a thermometer into the thickest part of thigh; it should read 165-170°C. Alternatively, cut into the leg joint; if the juices run clear, it's ready. Transfer the turkey to a large serving platter and rest under two layers of foil and a tea towel until ready to carve. Reserve 125ml (4fl oz) of the roasting juices for the gravy, skimming off any fat. To make the gravy, scrape up any burnt bits from the bottom of the roasting tin, then set it on the hob over a low heat. Pour in the roasting juices, then add the thyme and reserved chopped shallots. Cook for 2-3 minutes, then add the flour and whisk for 2-3 minutes until smooth. Add the mustard, then pour in the sherry and stock. Cook for a further 8 minutes, whisking continuously, until the gravy has thickened. Season well. To serve, garnish the turkey with the roasted shallots and the sage leaves. Serve with the gravy and all the trimmings.