CRUST: Sift flour and icing sugar together. Stir in lemon zest. Cut in butter with a pastry blender until mixture resembles coarse crumbs. Spoon into a greased 9-inch round pie plate or tart tin; flatten evenly with the back of a spoon. (NOTE: You can substitute with you favorite shortcrust pastry).
Bake crust for 20 minutes at 375F until slightly golden.
FILLING: Whisk together eggs, sugar, flour, lemon juice, lemon zest, and lemon oil, (if using) until frothy; pour mixture over the hot crust. (If the crust has developed air pockets, press them down with the back of a spoon before adding the filling).
Return to the oven for an additional 20-25 minutes, or until light golden brown. Cool on a wire rack.
RASPBERRY SAUCE: Process the berries in blender until smooth. Add sugar and liqueur, if using. Stir until sugar dissolves.
Cut tart into wedges, just before serving, drizzle with raspberry sauce on top,and garnish plate with raspberries. Dust the top with icing sugar.