Preheat the oven to 400F/200C
Chop the rhubarb into ¼ inch pieces and toss it with sugar, flour and lemon zest.
Roll out your pastry to fit the size of your pan. I used 9"X13" shallow pan. You should have two rectangular sheets, the bottom one slightly bigger than the bottom one.
Line the bottom of the pan with the first pastry sheet so the edges come up the sides of the pan and fill it with the rhubarb, sugar and flour mixture.
Cover with the top pastry sheet and crimp the edges. (You can cut various designs with cookie cutters if you have leftover pastry to decorate the top.)
Brush with lightly beaten egg, cut slits on the top for the pie to vent and sprinkle with a tablespoon of demerara/turbinado sugar.
Put in the preheated oven and immediately turn the temperature down to 180C/350F and bake for 45 minutes until the crust is golden and the filling is bubbling through the slits. Let the pie cool for 1 hour before serving. (This helps with the filling to be less runny.)
Serve with whipped cream or ice cream.