Bring a pan of salted water to the boil. Slice the unpeeled aubergines in thin rounds and drop immediately into the boiling water. Boil for 4-5 minutes, until the slices have just softened. Drain and lay the slices in a single layer between 2 towels to dry them thoroughly.
Cut the ends off the chicory and slice thinly lengthways. Spread 40 g/1½ oz of the butter over the bottom of a 24 cm/9½ in glass or earthenware flan dish and sprinkle evenly with the sugar. Then sprinkle with the citrus rind, cumin seeds and some salt and black pepper. Lay the long slices of chicory evenly on this mixture. Then lay on top of this a layer of aubergine slices, packed closely together. Continue laying on the rest of the aubergine slices, pressing them down evenly.
Melt the remaining 25 g/1 oz of butter and put on one side. Lay one sheet of filo pastry over the aubergines, turning in the edges to fit the dish. Brush all over thinly with melted butter. Then lay on another sheet of filo at a different angle so the edges even up. Brush again with butter. Continue like this with the other sheets of filo. Finally, prick the pastry all over with a fork. If not ready to cook, refrigerate the tart.
To cook, preheat the oven to 220°C/425°F/gas mark 7 and put the tart in just above the centre. Bake for 20-30 minutes, until the pastry is a rich golden brown. Turn down the heat to 170°C/325°F/gas mark 3 and bake for a further 25-30 minutes.
Remove from the oven, leave for 5 minutes, then loosen the sides carefully with a knife. Put a large, flat serving plate on top of the flan dish and turn them both upside down. Give a shake and turn the tart on to the serving dish. Serve warm.