Mix the flour with the butter and salt until the mixture forms into floury breadcrumbs. Then add cold water until the mixture comes together into a ball of dough – a few tbsp should be enough. Rest the pastry wrapped in clingfilm in the refrigerator for 20 minutes.
Preheat the oven to 180°C/350°F/Gas 4 with a baking sheet in it to heat it up.
Roll out the pastry onto a lightly floured surface, then line a lightly greased flan tin with it. Press the pastry gently down into the tin then prick all over with a fork. Bake in the oven for 20 minutes until pale gold.
Meanwhile heat the butter in a pan and fry the onion until softened and light gold. Stir in the tuna and then spread the mixture over the pastry base. Whisk together the eggs, yolk and double cream and a little salt and pepper and pour over the tuna mixture. Gently arrange the sliced tomatoes on top of the quiche.
Transfer the flan tin to the baking sheet and bake for 35-40 minutes until the centre is set and the filling golden and puffed up. Serve warm.