Mix the flour, sugar, salt and lemon zest in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Add the egg, mix quickly to form a soft dough – do not knead. Flatten the dough, cover and chill for approx. 30 mins. On a lightly floured surface, roll the dough out into a rectangle of approx. 42 x 18 cm, transfer to the prepared tray. Prick firmly, refrigerate for approx. 30 mins.
To blind bake
Cover the dough with baking paper, weigh down with dried pulses. Bake for approx. 20 mins. in the lower half of an oven preheated to 220°C. Remove from the oven, allow to cool slightly, remove the baking paper and pulses, remove from the tray and leave to cool completely on a rack.
Bring the water and sugar to the boil in a wide pan, poach the rhubarb in batches for approx. 2 mins., carefully remove using a slotted spoon, place on a tray.
Line up approx. 4 ribbons of cooked rhubarb side-by-side on a sheet of baking paper. Weave one ribbon after the next crosswise over and under the vertical ribbons to create a lattice pattern. Repeat until the lattice is the same size as the tray, place any cut-offs in a measuring cup and set aside.
Add the quark, icing sugar and vanilla to the rhubarb cut-offs, puree. Fold the whipped cream into the rhubarb and quark mixture, spread over the tart base. Place the rhubarb lattice over the filling. Cover the tart and refrigerate for approx. 30 mins.