Peel the potatoes and boil until tender, then drain and mash them with the cream, 50 g/2 oz of the cheese and the grated nutmeg. Whisk the eggs until frothy and mix them with the mashed potatoes. Season well with salt and plenty of black pepper.
Preheat the oven to 180°C/350°F/gas mark 4. Peel the onions and chop up fairly small. Cut up the turkey into small pieces, either briefly in a food processor or by hand. Put the tomatoes into a bowl, pour boiling water over them and leave for a minute or two, then drain, peel and chop up roughly.
Melt 50 g/2 oz of the butter in a large, deep frying pan, add the onions and stir over a fairly high heat until they begin to brown. Stir in the cinnamon and remove from the heat. Turn the onions and butter into a large bowl. Roughly chop the sage leaves and mix in, together with the turkey and tomatoes. Season with salt and black pepper. Transfer to a large, fairly shallow ovenproof dish. Spread the mashed potatoes over the top, dot all over with the remaining butter and sprinkle with the remaining grated cheese. Cook in the centre of the oven for 50-60 minutes, until richly browned.