In a large bowl, mix the plain flour, 50 g caster sugar and a pinch of salt.
Add the cold unsalted butter cubes and rub in with your fingertips until the mixture resembles fine breadcrumbs.
Stir in the egg and bring the mixture together, kneading lightly until it forms a smooth dough.
Wrap the dough in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 180°C (160°C fan/Gas Mark 4) and grease and lightly flour a 22–24 cm loose-bottomed tart tin.
On a lightly floured surface, roll out the pastry to fit the tin. Transfer it to the tin, pressing into the edges, trim any excess and prick the base with a fork.
Peel, core and thinly slice the apples.
In a bowl, toss the apple slices with 50 g caster sugar, the cinnamon and the lemon juice.
Arrange the apple slices over the pastry base and drizzle over the melted butter.
Bake for 40–50 minutes until the pastry is golden and the apples are tender. If it browns too quickly, cover with foil.
Meanwhile, warm the apricot jam with the water until runny, then brush it over the tart to glaze.
Leave to cool slightly before removing from the tin. Serve warm on its own, or with vanilla ice cream or whipped cream.