With an electric whisk, beat the butter and sugar until pale and fluffy.
Add the lemon juice and rind, following by the egg yolks, one at a time until well blended- if the mixture looks a bit curdled at this stage don't worry.
Sift over the flour and stir in .
Rinse of the beaters then whisk the egg whites until they form soft peaks; don't allow them to get to stiff or they will be difficult to fold into the mixture and you'll end up losing all the air you've just beaten inches.
With a large metal spoon gradually fold the egg whites into the blueberry mixture.
Pour into the dish, place on a baking sheet and bake for 40-45 minutes until is risen, golden and firm to touch.