Add half the sugar and the water to a saucepan and heat gently until the sugar has dissolved. Add the orange slices and cook over a low heat for 30 minutes until the oranges are tender, the peel is soft and almost translucent and most of the syrup has evaporated. Cool.
Roll the pastry out thinly on a lightly floured surface to fit a buttered 24 cm (9½ inch) fluted loose-bottomed tart tin. Press the pastry over the base and sides of the tin. Trim off the excess with scissors so that it stands a little above the top of the tin. Chill for 15 minutes.
Bake the tart blind (see
Cream the butter and the remaining sugar in a bowl until light and fluffy. Add the ground almonds, gradually beat in the eggs until smooth, then set aside.
Reserve 10 of the best orange slices for decoration, then drain and chop the rest. Stir the melted chocolate into the almond mixture, then mix in the chopped oranges. Spread into the tart case, arrange the reserved orange slices in a ring on top and bake for 30 minutes. Leave to cool for 30 minutes before removing from the tin.