Put the sun-dried tomatoes in a small heatproof bowl and pour over enough boiling water to cover. Leave to soak for 20 minutes.
Meanwhile, put the leeks and wine in a saucepan and bring to the boil, then reduce the heat and simmer until the liquid has evaporated. Remove the pan from the heat and stir in the milk, egg yolk and cheese. Season with salt and pepper.
Beat the egg white lightly in a jug. Brush a filo pastry square with a little of the egg white and use it to line the base and sides of a 10 cm (4 inch) tart tin. Brush 2 more filo squares and lay these on top, each at a slight angle to the first, allowing the edges to overlap the rim. Repeat with the remaining squares to line 3 more tart tins.
Place a spoonful of the cooked leek mixture in each pastry case. Lay 2 of the drained, rehydrated tomatoes on top of each tart and cover with the remaining leek mixture. Season again and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes, covering the tarts with pieces of foil after 10 minutes. Serve hot, with cherry tomatoes and sliced red onion if liked (remembering to count the extra calories).