Remove the tin and turn the oven oven up to 180C/160C fan/gas 4.
In a bowl measure out the sugar, flour and zest. In a jug add the eggs and lemon juice and whisk until combined.
Then sieve into a bowl with the sugar, flour and zest. Whisk to combine then pour over the base.
Bake for 25 until the top is just set. Cool in the tin, dust with icing sugar, then slice.