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Heat oven to 200C/fan 180C/gas 6. On a floured board, roll out the pastry to about the thickness of a £1 coin. Press into a 20cm tart tin with a removable bottom. Line the pastry with greaseproof paper and fill with baking beans. Bake for 20-25 mins until the sides of the pastry turns golden brown. Remove the baking beans and greaseproof paper and prick all over the base and sides of the pastry with a fork. Put the tart shell back into the oven for another 10 mins until the base is cooked and the pastry is biscuity brown all over. Leave to cool on a wire rack.
In a saucepan, cook the onions in the olive oil for 5 mins until soft. Season with salt and pepper if you like, and set aside to cool. Stir together the lemon juice and crème fraîche, and then set aside.
Assemble the tart: spread the onion over the base and arrange the smoked salmon pieces on top. Sprinkle over the capers, dill and lemon zest. Keep the tart in its tin, cover with cling film or foil and refrigerate until serving, drizzled with crème fraîche.