Heat oven to 220C/200C fan/gas 7. Unroll the pastry and cut it into 4 rectangles. Score a border about 2cm in from the edge of each piece, then lift onto a lightly floured baking sheet. Mash the goat’s cheese with the herbs, plus some seasoning. Spread over the middles of the tarts.
Overlap the courgette and tomato slices on top of the cheese, then scatter with the olives and thyme leaves. Drizzle with a little olive oil. Brush the edges of the pastry with egg. Bake for 15-20 mins until the pastry is dark golden and the vegetables are starting to sizzle.
Whisk together the oil, vinegar, mustard and honey to make a dressing, then season to taste. Toss with the watercress just before serving.