Preheat the oven to 230°C/gas 8.
Roll out the puff pastry to 3mm thickness and cut into 4 even-sized circles, each the size of a side plate. Place the puff pastry circles on a greased baking sheet.
Brush the edges of the puff pastry circles with beaten egg. Press the edges with a fork.
Spread the mustard, then layer the sliced mozzarella evenly over the pastry circles, leaving an edge of pastry around the outside.
Arrange the sliced tomatoes in a circle in the centre of each pastry circle. Season with salt and freshly ground pepper, sprinkle with thyme and drizzle with olive oil.
Bake the tomato tarts for 15-20 minutes until the pastry is cooked through.
Meanwhile, prepare the salad. Mix together the balsamic vinegar and olive oil. Season with salt and freshly ground pepper. Toss the salad leaves with the salad dressing.
Transfer the baked tomato tarts to a warm serving plate. Drizzle the pesto over the tarts and arrange a small pile of dressed salad in the centre of each tart. Serve.