Make sure you buy British - the tomato season's in full swing in summer so there are plenty of varieties to choose from
Heat the oven to 200C/fan 180C/gas 6. Unroll the pastry onto a baking sheet and score a border 1cm from the edge. Prick inside the border with a fork, then bake for 15–20 minutes until light golden.
Gently squash down the middle. Spread the pesto inside the border, dot over the mascarpone, then layer the tomato on top. Season. Bake for 10 minutes until the tomatoes are cooked and the marscarpone has melted.