Roll out the larger piece of pastry thinly (about 2mm thick) on a lightly floured surface, into a large rectangle and use to line a 20x30cm tart or swiss roll tin. Press the pastry right into the corners and leave about a cm overhanging, adding any scraps to the other ball of pastry.
Spread the frangipane evenly over the base of the pie and top with the rhubarb compote, making sure that they are both level.
Roll out the remaining pastry thinly and cut into thick and thin strips, some the same length as the pie and some the same width. I used three 3cm wide and three 1cm wide strips to go length-ways and four of each to go crosswise. Re-roll any pastry scraps to cut more strips as needed.
Lay the length-ways strips across the pie, alternating between thick and thin. From the centre of the pie, fold back every other wide pastry strip and neighbouring thin strip. Lay down one wide pastry strip and one thin strip in the opposite direction, then fold the pastry back over the top.
Alternate the strips you’ve folded back and repeat the process of laying down the pastry strips in the opposite direction and folding back over to create the woven lattice pattern.
Repeat this over the other half of the pie, going in the opposite direction.
Use some of the reserved egg white to glue the lattice strips to the base of the pie then trim the edges with a sharp knife. Place the pie in the freezer for 20 minutes while you heat the oven to 200C/400F/gas mark 6 and place a baking tray into the oven to heat up.
Brush the top of the pie with egg white and sprinkle with demarera sugar. Place the tin on the hot baking tray (helps prevent a soggy bottom) and bake for 60-80 minutes until golden brown and bubbling. You will probably need to cover the top of the pie with tin foil partway through baking to prevent the top from catching before it is cooked in the middle.
Allow the pie to cool in the tin for half an hour before carefully turning it out and serving warm with ice cream or custard.
