Divide the pastry into quarters and roll each out on a lightly floured surface until 2 mm (⅛ inch) thick. Using a 13 cm (5½ inch) plate as a guide, cut out 4 rounds – make a number of short cuts around the plate rather than drawing the knife around, which can stretch the pastry. Place the rounds on a baking sheet.
Place a slightly smaller plate on each pastry round and score around the edge to form a 1 cm (½ inch) border. Prick the centres with a fork and chill for 30 minutes.
Arrange the apple slices in a circle over the pastry rounds and sprinkle with the sugar. Grate the butter over the top and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 25–30 minutes until the pastry and apples are golden.
Meanwhile, make the apricot glaze. Put the jam in a small saucepan with the lemon juice and water and heat gently until the jam melts. Increase the heat and boil for 1 minute, remove from the heat and press through a fine sieve. Keep warm then brush over each apple tart while they are still warm. Serve with ice cream.