Line a large baking sheet with nonstick baking paper. Roll out the pastry on to a lightly floured surface to a 35 cm (14 inch) square, trimming the edges if necessary, to make a neat square. Place on the baking sheet and scrunch up the edges of the pastry to prevent the filling running out.
Toss the apples in the lemon juice. Dot some of the butter over the base of the pastry and sprinkle with 1 tablespoon of the sugar.
Arrange the apples in neat rows on the pastry, then dot with the remaining butter and sprinkle over the remaining sugar.
Place in a preheated oven, 220ºC (425ºF), Gas Mark 7, for 15–20 minutes or until golden and crisp.
Warm the apricot jam in a small saucepan, then brush over the apples and pastry. Serve immediately with scoops of vanilla ice cream, if using.