Preheat the oven to 200C/400F/gas 6. To make the pastry, place the flour, butter and salt in a food processor and pulse until the butter is incorporated but leaving some clumps. Turn on the food processor and add the egg, then add a drizzle of cold water or milk if needed until the dough just starts to come together. Take out of the mixer, shape into a ball and wrap in cling film. Place in the fridge to chill for up to 30 minutes (you can skip this step, but for the best pastry, do leave it to rest).

Roll out the pastry on a clean floured surface until it's thin and will cover a lightly greased 23cm tart tin with some overhang. Use clean fingers to gently push the pastry into the tin, leaving it hanging a little over the edges. Prick the base lightly all over with a fork. Cover with tin foil and baking beans (or uncooked beans/rice) and blind bake in the oven for 10 minutes. Remove the beans and foil and return to the oven for another 5 minutes, until just cooked and starting to go ever so slightly golden. Trim any overhanging pastry and allow to cool a little.

Spread the mustard thinly on the base of the pastry case, then layer slices of tomato and mozzarella, slightly overlapping each other, all over the pastry case. Lightly drizzle with extra virgin olive oil and sprinkle with mixed herbs, if you like, then return to the oven for 30-40 minutes, until the cheese is golden, pastry is browned, and the tomatoes and soft. Sprinkle with fresh basil leaves and serve warm or cold with a green salad and/or freshly made coleslaw. Cooked vegetables, cured meats and olives also go beautifully with this.