1. Preheat the oven to 200ºC, gas mark 6. Place the flour in a large bowl and, using your fingertips, rub in the butter until finely blended. Make a well in the centre and pour in the milk, then use a round-bladed knife to bring it all together to make a soft dough.
2. Knead the dough briefly then roll out and use to line a 23cm loose-bottomed tart tin.
3. Beat together the egg, soft cheese, mustard and three-quarters of the Wensleydale and chives. Spoon into the tart case.
4. Arrange the tomatoes on top, cut-side up, then scatter over the reserved Wensleydale. Grind over some black pepper and spray lightly with olive oil.
5. Sit the tart on a baking tray and bake for 40 minutes until set and golden. Sprinkle over the reserved chives and serve with a watercress salad.