Make pastry. Chill at least 1/2 hr before serving. Use flan tin with removable bottom. Preheat oven to 375F. While pastry is baking, cream together butter & sugar till light & fluffy. Add eggs, one at a time, beating after each addition, add lemon zest and beat again. Pour in orange and lemon juices and beat again. Set aside. When tart has cooled a little, patch any holes if necessary. Beat filling again till smooth and bake for 25-30 minutes or till filling is set. If browning too much, cover edges with foil. Cool and add lemon decoration if you wish. Lemon decoration: While tart is baking slice lemon into thin rounds. Bring 1/2 cup water & 1/2 cup sugar to a boil. Add lemon slices and simmer gently for 10 minutes. Remove from heat and allow to cool in syrup. When tart has cooled some, arrange lemon slices on top,put icing sugar in small sieve and sprinkle over top. It will melt on the lemon slices so that they show through.