Heat oven to 170C/150C fan/gas 3. Pour water, flour, butter, salt, egg and baking powder into the bowl. Knead the dough for a few minutes until is soft and not sticky. Roll out the dough on a well-floured surface to a 1cm thickness. Place the dough in a flan case then on a baking tray. Prick the bottom of the tart with a fork then bake in oven for about 20 minutes.
Make syrup by melting the butter with lemon juice and sugar in a small saucepan. Remove from the heat.
Cut peeled and cored apples/pears into thin slices. When the tart bottom has cooked, lay the apple/pear slices in concentric circles. Sprinkle the syrup over the apples/pears. Bake for 20 minutes until the pastry is pale golden brown.
Whisk together the double cream, egg yolks and sugar till thick and creamy. When the pastry is ready pour the cream over the tart. Bake in the oven for 10 minutes until the mixture is just set.