Preheat the oven to 180C/350F. Line the 9X13 inch baking pan with parchment paper, so the ends overhang. Set aside till needed.
In a food processor combine the flour, cubed butter and the icing sugar, pulse until the mixture resembles crumbs but forms a clump if you grab a handful and squeeze. (See pictures below for visual guidance.) Alternatively, you can achieve the same texture without a food processor by cutting the butter into the dry mixture with a pastry cutter.
Cover the bottom of parchment lined pan with the crumbs and press firmly. Bake in the preheated oven for 20 minutes. It will look pale and not baked.
While the shortbread crust is baking make the lemon filling. In a large bowl zest the lemons and mandarin oranges. Then squeeze the juice from both in a separate small bowl until you have 155 ml/2/3 cup. Add the juice to the bowl with the zest, then add the eggs and beat the mixture with a whisk, sift in the flour and continue whisking to prevent lumps forming, then add the baking powder, salt and granulated or caster sugar, mix well until he mixture is well blended.
Take the shortbread out of the oven and lower the temperature to 160C/325F. Pour the lemon filling into the hot crust, which you previously gave a good mix with a whisk. Put it back in the oven and bake for 20-24 minutes or until the mixture is just set. Take it out of the oven and sprinkle with 1 tbsp of icing sugar. Set the pan on a wire rack and cool completely. I recommend cooling it in the refrigerator overnight for the best texture.
Then take the whole slab out of the pan by pulling the overhanging ends of parchment paper and cut into squares with a sharp knife. You might want to wipe the knife with paper towel in between cuts to maintain clean edges.