1.Preheat oven to moderate, 180ºC (160ºC fan forced). Line an oven tray with baking paper.
2.Cut each pizza base into quarters lengthwise. Then cut corners at an angle to create a coffin shape.
3.Spread each piece with pesto. Top with potato and mushroom slices. Transfer to tray. Drizzle with olive oil and season to taste. Scatter over parmesan and thyme sprigs. Bake 15-20 minutes, until crisp and golden.
These pizza bases measure around 20 x 30cm and are available from the bakery section of your supermarket. If unavailable use shortcrust pastry. Parboil potatoes by cooking them whole in a saucepan of simmering water for about 5 minutes, until just tender.