Roll out the pastry to about 5mm thick. Place in the bottom of a 20x30cm slice tin. (I usually line the bottom of the tin with baking paper just to be sure it comes out easily.)
Preheat the oven to 190 degrees C. Using a fork poke the pastry then cover with baking paper and place some dried beans over top. Blind bake the pastry for 15 minutes.
Remove the baking paper and beans from the baked pastry. Bake for a further 5 minutes until golden in colour and set aside to cool.
Spoon the raspberry jam over the cooled pastry base and spread evenly.
To make the filling beat together the butter and sugar until pale and fluffy. Add the eggs one at a time beating after each addition. Add the lemon juice, lemon zest and the almond essence. Fold in the almond meal.
Spoon the mixture over the jam base and spread to the sides of the tin. Sprinkle the slivered almonds over top.
Bake in the preheated oven for 30-35 minutes until the filling is golden on top and feels firm in the middle. If the pastry starts to get too brown cover with foil.
Remove and allow to cool. When completely cooled dust with icing sugar and cut into slices.