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Put the flour and salt into a bowl and rub in the butter until the mixture resembles coarse breadcrumbs. Add the egg and, using a knife, stir in just enough cold water to bind the dough together.
Tip the dough on to a lightly floured work surface and shape into a ball. Cover with cling film and chill for at least 30 minutes, or ideally 1-3 hours, before using.
Preheat the oven to 180°C/350°F/Gas 4, placing an oven rack in the lower part of it. Grease a 30cm loose-bottom tart pan.
Roll out the pastry on a lightly floured work surface until it is large enough to line your tart pan.
Spread the mustard on the bottom of the pastry case. Place some sliced tomatoes around the edge of the case, making sure they overlap, then fill in the middle. Sprinkle the cheese over the tomatoes, season with salt and pepper, then drizzle with the oil.
Place the tart on a baking sheet and bake on a low oven rack for 40 minutes. Take the tart out of the oven and carefully slip off the outer ring, leaving the tart sitting on the base. Return to the oven on the baking sheet for another 5–10 minutes so that the sides get really crisp.