Preheat the oven to 240°C (475°F/Gas 8). Place the pizza base on a baking tray dusted with polenta and spoon the tomato sauce onto the base, spreading it up to the rim. Scatter 60 g (2¼ oz) chopped mozzarella cheese over the pizza. Visually divide the pizza into quarters. Put 30 g (1 oz) sliced prosciutto on one quarter, 4 quartered artichoke hearts on another, 4 sliced mushrooms on the next and 1 sliced tomato on the last quarter. Bake for 12 minutes, or until the crust is browned and the centre is bubbling.