Peel the pumpkin, cut out the seeds and cut the flesh into very thin, long slices about 2 mm(1/16 inch) thick. Put the slices in a colander, sprinkle with salt and leave for about 1 hour. Rinse them very well and pat dry. Preheat the oven to 180°C (350°F/Gas 4).
Drizzle 2 tablespoons of olive oil into a round 25–30 cm(10–12 inch) baking dish and spread it to coat the base of the dish. Put the flour on a plate and pat both sides of the pumpkin slices in it. Make a slightly overlapping layer of slices in the baking dish. Trickle the tiniest bit of olive oil over this layer, then repeat the layering and oiling until you have used up all the pumpkin (you should have four or five layers).
Mix a little salt into the passata and dot here and there over the top of the pumpkin. Put into the oven and bake for about 50 minutes, until the bottom is sizzling and the top is turning quite golden. Scatter the oregano over the top, crushing it between your fingers. Sprinkle with the mozzarella and return to the oven for 5 or 10 minutes, until the cheese melts and browns slightly. Cool a little before serving in wedges like pizza.