Take the pastry out of the fridge and allow it to come to a workable temperature. You should be able to mould it with your hands and it shouldn’t crack when you roll it out. That said, you never want it to become too warm. If the butter starts to melt at any point, pop it back in the fridge to cool down.
Preheat the oven to 200 degrees Celsius.
Lay the pastry on a large sheet of baking paper that has been liberally dusted with tapioca flour. Roll it out gently (flipping it intermittently to ensure it hasn’t stuck) to a large rectangle. The size doesn’t matter too much, but eating thick pastry isn’t always pleasant so keep it around 1/2 a centimetre or less.
Once you have the pastry in a nice rectangular shape, flip the edges of the pastry up over themselves to create a crust or a lifted edge. Once you’ve done this all the way around the tart, liberally flour the top, and use a second sheet of baking paper to gently flip it over. You should have a lifted crust around the edges, seam side on the bottom of the tart. Use your hands to define the crust so no tomato falls out.
Dust off any excess tapioca flour if there is a lot. It does help absorb extra liquid though, so leave a little on there (particularly if you have watery tomatoes.)
Finely grate gruyere over the base of the tart, and season with sea salt flakes and freshly cracked pepper. Arrange the tomato slices over the top of the gruyere. You can douse the tomatoes in a little olive oil if you fancy.
Paint the edges well with egg wash before placing the tart in the oven for 20 minutes. After 20 minutes is up, give the pastry another quick paint of eggwash. Turn the oven to the grill setting. If you’re adding mozzarella, now is the time to do it.
Return the tart to the oven and allow to cook for 5-10 minutes, watching closely to ensure nothing burns. When the pastry is golden brown to your liking and the mozzarella melted, remove the tart from the oven. Drizzle some pesto over the top before finishing with fresh basil leaves and a little extra seasoning if necessary. Best served while warm, but if you’re serving it cold, wait to add the pesto and basil immediately before serving.