To fill one 9in square cake tin - about 30-35 full pieces
First make the syrup: put the water and sugar and lemon juice in a heavy bottomed pan and bring to the boil. Keep it boiling for 7 minutes to get a reasonably thick but still definitely pour-able syrup. It should not start to colour. Remove from the heat, add the rosewater and stir to combine. Let it cool slightly in the pan, then decant to a jug to finish cooling.
Preheat the oven to 190°C. Line the base and sides of the cake tin with greaseproof or baking parchment. Grind the walnuts to a coarse, nubbly powder. It shouldn't be so far gone that it is flour, and definitely not becoming at all greasy.
On a lightly floured surface, roll out the puff pastry to an oblong 9 inches wide and 18 inches long, i.e. twice the area of the cake tin. Cut in half. Lay one piece of pastry in the bottom of the tin. Brush liberally with melted butter. Tip the ground nuts over this base and spread out evenly. Pat down fairly firmly. Lay the remaining sheet of pastry on top and brush again with butter. (You will probably have some left over.) Cut diagonal lines about 3cm apart to make even diamonds, making sure to cut all the way through the base layer. Bake in the preheated oven for about 10 - 15 minutes, until the top is an even golden brown. When you remove it from the oven, pour over half of the syrup and let it sink in, then add the rest. Once completely cold, slice through again following your original pattern, remove from the tin and enjoy!