To make the dough, tip the flour into a large bowl and make a well in the middle. Add the sourdough starter and water, mix to bring together, adding the salt just before it is fully mixed. Turn the dough out onto a clean work surface and knead until it becomes smooth.
Roll the dough into a ball and place in a lightly greased bowl. Cover with cling film and leave at room temperature for 5 hours or until it has doubled in size.
2 hours before cooking, move the dough onto a large, oiled baking tray, keeping it the same way up. Cover with cling film and leave to prove for another 2 hours.
Preheat the oven to 220C/200C Fan/Gas 7 and grease two baking trays with oil.
To make the tomato-based pizza, stretch half the dough with your hands to a 28cm/11in disk and place on a greased baking tray. Spread it with 1 tablespoon of the crushed tomatoes and cook for 10 minutes. Tear over the fior di latte and cook for a further 3–5 minutes.
Put the basil leaves in a small bowl with the oil, lemon juice and salt.
Scatter the dressed basil leaves over the cooked pizza, cut into 8 and serve.
To make the white-based pizza, stretch half the dough with your hands to a 28cm/11in disk and place on a greased baking tray. Tear over the fior di latte and cook for 10 minutes.
Meanwhile, preheat the grill to its highest setting and grill the pak choi for 2 minutes on each side. Dress with a little olive oil and salt.
When the pizza is cooked, top with the pak choi, smoked ricotta and chilli. Cut into 8 slices and serve.