Preheat the oven to gas 6, 200C, fan 180°C Put the flour, butter and salt in a food processor and pulse until the mixture resembles breadcrumbs. Add the yolk and 4tbsp cold water, pulse until the dough comes together (add a little more water if needed). Turn onto a floured work surface and knead gently. Roll out to roughly 1/2 cm (1/4 in) thick.
Line a 25cm (10in) loose-bottomed tart tin evenly with the pastry, leaving an overhang. Prick the base all over with a fork, then transfer the tin to a baking tray and chill for 10 minutes.
Take the pastry case out of the fridge and line it with nonstick baking paper, then fill with baking beans and blind bake for 15 minutes. Remove the beans and the paper, then return to the oven for a further 8-10 minutes, or until golden in colour. When the pastry is cool to the touch, trim the edges with a knife.
Meanwhile, heat 1tbsp of oil in a nonstick frying pan and fry the bacon for 2-3 minutes each side, or until golden, then remove and set aside. Add the remaining oil to the pan and cook the whole mushrooms for 3 minutes, until softened. Set aside.
Spread the ketchup over the base of the quiche, then top with the tomato halves, cut side up, the mushrooms, and the bacon.
Beat the eggs together with the double cream and then season with salt and pepper. Pour the mixture into the case and bake for 25-30 minutes until the egg is set and the top is golden in places.