Preheat oven to 200°C. Put flour and a quarter of a cup of the parmesan in the bowl of a food processor. Add butter and process until mixture resembles fine breadcrumbs. Add cold water and process until mixture begins to form a ball. Turn out onto a floured surface and knead for about 30 seconds until smooth.
Cut dough into three even pieces. Roll out each piece separately on a floured surface to a 38cm x 20cm rectangle (5mm thick). Dust a large oven tray with flour then place dough on tray.
In a small jug, combine oil, garlic and thyme, then brush dough with oil mixture. Season.
Sprinkle with a quarter of a cup of the parmesan. Place a third of the onion slices evenly over dough then top with a third of the sliced camembert and torn prosciutto. Brush lightly with oil mixture.
Bake, for about 15 minutes, turning tray once, or until golden. Repeat to make two more flatbread, bake last two together, swapping trays halfway. Allow to cool for 10 minutes. Brush lightly with remaining oil mixture just before serving. Season and cut into pieces. Serve.