Place flour and butter in a food processor and process until mixture resembles coarse breadcrumbs.
Add icing sugar, oil, egg yolk and 1 1/2 to 2 tablespoon cold water and process until mixture just forms a ball.
Turn out onto a lightly floured surface, knead lightly and shape into a disc and roll out to a 25cm round and ease into a 2cm deep 23cm loose base flan pan and trim edges and lightly prick base with a fork and chill for 10 minutes.
Preheat oven to 190C to 170c fan force and place pan on a baking tray and line pastry with baking paper and fill with pastry weights, rice or dried beans and bake for 15 minutes and then remove paper and weights and bake for another 5 minutes or until pale golden in colour.
Meanwhile, to make filling, whisk eggs and eggwhites until well combines.
Sift icing sugar into a bowl and then gradually whisk in egg and stir in lemon rind and juice.
Strain through a fine sieve and add sour cream and whisk until smooth.
Carefully pour into warm pastry case.
Reduce oven to 150c or 130c fan forced and bake tart for 25 to 30 minutes until just set and still wobbly in centre.