Mix the flour, nuts, sugar and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Add the egg, mix quickly to create a soft dough – do not knead. Roll out the pastry on a lightly floured surface, place in the prepared tin, prick the base firmly, cover and chill for approx. 30 mins.
To blind bake
Approx. 15 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly on a cooling rack.
Using the whisk on a hand mixer, beat the butter, sugar and salt, add the eggs, beat for approx. 3 mins. more until the mixture is lighter in colour. Blitz the nuts in a food processor until the mixture is creamy, stir into the mixture with the flour, spread over the tart base.
Place the rhubarb in a diamond pattern over the nut filling, sprinkle over the sugar.
Approx. 1 hr. in the lower half of the oven. Remove from the oven, allow to cool slightly, slide the tart onto a cooling rack, leave to cool completely.