Preheat oven to Gas Mark 7, 220ºC, fan 200ºC. Unroll the pastry onto a nonstick baking tray and score a line 3cm (1in) from the edge all round. Spread the centre of the pastry with the mustard and sprinkle with half the cheese.
Cut the tomatoes in half and scatter over the top of the tart. Sprinkle with the remaining cheese. Bake the tart in the oven for 15-20 minutes or until golden brown. Sprinkle with the basil leaves, then serve warm or cold.
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