Cook the apples, water, sugar, lemon zest and juice gently for 8–10 minutes until just tender.
Stir through the cornflour mixture and cook for 1 minute.
Cool completely.
Beat butter and sugar until pale and creamy.
Beat in the egg.
Add sifted flour, baking powder and salt and mix gently to form a soft dough.
Divide dough in half.
Roll each piece between sheets of baking paper to fit the tin.
Place one piece, still on its bottom sheet of paper, into the tin.
Spread over the cooled apple.
Top with the remaining rolled dough and sprinkle generously with sugar.
Bake for 35–40 minutes, until beautifully golden.
Cool before slicing.
