Makes four tarts
1 Heat the oven to 220C/Gas Mark 7. Line a baking sheet with baking paper.
2 On a lightly floured worktop, roll out the puff pastry until it is about 2cm thick. You will need to cut four circular discs from the pastry, so find a small plate or saucer that’s about 16cm in diameter to use as a template.
3 Place the plate over the pastry and run a sharp knife around the edge to cut out the first disc. Repeat until you have four.
4Now you need to lightly score a margin about 1.5-2cm in from the edge of the pastry discs, so find a smaller plate or bowl that you can use as a template. Place the smaller plate in the centre of each pastry disc and lightly run a sharp knife around the edge, being careful not to cut all the way through.
5 Lift the discs onto a baking sheet and brush with the beaten egg. With a fork, prick the inner circle but not the outer edge – this will stop the pastry from rising in the middle, but will allow the outer edge of the tart to rise up.
6 Take a good amount of the pesto and spread it over the inner circle of each tart. Place the tops and bottoms of the tomatoes in the centre of the tarts, then start to lay the slices on top of the pesto so they overlap and create a swirl effect – it should look a bit like a Catherine wheel. Season with salt and pepper.
7 Place the tarts in the oven and bake for 20-25 minutes, until well risen and coloured. Mix the watercress with the olive oil and serve with the tarts.