Preheat the oven to Gas Mark 6, 200°C, fan 180°C. Peel, core and roughly chop 8 apples, then toss in half the lemon juice. Place in a medium saucepan, pour over the sugar and vanilla extract, and simmer for 15 minutes until soft. Mash lightly with a fork.
Line a loose-bottom 35cm x 21cm (13in x 8in) fluted tart tin with the pastry and trim off the excess with a sharp knife. Prick the base with a fork, and chill for 15 minutes. Cover the pastry with nonstick baking paper and fill with baking beans. Bake for 15 minutes before removing the paper and beans and cook for a further 5-10 minutes until golden. Leave to cool slightly.
Whisk the eggs into the apple purée, then spoon into the tart case. Core and thinly slice the remaining apples, toss in the remaining lemon juice, then fan the apple pieces out over the purée. Transfer to the oven to bake for 30 minutes, until golden.
Heat the apricot jam in the microwave for 30-45 seconds, until runny, then brush thickly over the apple flan. Serve hot or cold.
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