Preparation
Preheat the oven to 220°C, fan 200°C, gas 7. Scrub, trim and peel the beetroot, then thinly slice into 5mm rounds. Toss with the oil, season and arrange in one layer on a large, lined baking tray. Roast for 20-25 minutes until tender. Meanwhile, put the spinach in a colander and set over the sink. Pour over a kettle of just-boiled water to wilt. Leave to cool, then squeeze out the excess water.
Roll the pastry on its paper onto a large baking sheet. Score a 2cm border around the edge and prick the base all over with a fork. Lightly beat the egg, then use to brush the border of the pastry. Bake for 10-12 minutes, or until golden and well risen.
Reduce the oven temperature to 200°C, fan 180°C, gas 6. Gently press down the centre of the tart where it has puffed up. Spread with the pesto for punch, then top with the wilted spinach and season with salt and black pepper. Tear over the mozzarella for creaminess and top with the roasted beetroot for earthiness. Bake for a final 10-12 minutes until golden and melted, then slide onto a board and scatter with the basil, if using, to serve.