Lightly oil a bowl with spray oil. Turn the dough out on a lightly floured work surface. Knead for 10 minutes until smooth and elastic. Put in the prepared bowl, cover and leave to rise in a warm place for 1 hour until doubled in size.
Meanwhile, cut the squash in half and scoop out and discard the seeds and fibres. Cut into 2.5 cm (1 inch) dice. Put in a roasting tin, add the onion, half the oil, the garlic, chilli flakes, sage and salt and pepper and toss well. Roast in a preheated oven, 230°C (450°F), Gas Mark 8, for 25 minutes until tender, stirring halfway through.
Put a heavy-based baking sheet on the middle shelf of the oven and heat for 5 minutes. Knock the air out of the dough. Divide in half. Roll one half out to a 25 cm (10 inch) round. Transfer to the heated baking sheet. Top with half the squash mixture and half the cheeses. Bake for 10–15 minutes until the base is crisp and golden. Serve immediately. Repeat to make the second pizza.