Pre-heat oven to moderate (180 degrees celsius, fan-forced).
Grease a slice tin (approx 28 x 18cm) and line with baking paper, leaving paper overhanging at sides to assist with removal of slice later.
Cream butter and sugar for biscuit base and layer.
Whisk the egg lightly and add to the mixture. Mix well to combine.
Fold in sifted flour and salt to form a pastry dough. Divide into two and set aside.
Make the filling by combing all filling ingredients and whisking together.
Make the meringue by beating egg white to form soft peaks and then gradually adding the sugar and beating to form stiff peaks. Fold the salt and cream of tartar into the meringue mixture.
To put the slice together, first roll out half of the pastry dough on a work bench dusted with flour. Place in the bottom of the slice tin, using a bread knife to cut the layer to fit the tin and using your fingers to spread and even out the pastry.
Pour lemon filling over the pastry layer.
Roll out the second half of the pastry dough and cut to the shape of the tin. You can do this by placing the tin on top of the rolled out pastry and cutting to shape with a bread knife.
Very carefully, roll half of the pastry onto a rolling pin and use a spatula to lift the remaining half of the pastry and place as the next layer in the slice tin.
Spread the meringue over the top of the pastry layer and use a spoon to create little peaks in the meringue.
Place in the oven and bake for 30-35 minutes.
Allow to cool in the tin and then lift out and cut into squares.