Bring the water to the boil in a large saucepan. Remove from the heat, then add the polenta in a slow, steady stream, stirring constantly with a wooden spoon. Put the pan back onto the heat and cook gently, still stirring, for 5 minutes, until very thick.
Remove the pan from the heat and stir in the corn, spring onions, oregano and freshly ground black pepper to taste. Transfer the polenta mixture to an oiled Swiss roll tin or rectangular baking tin measuring about 30 × 20 cm, and spread out evenly, smoothing the top. Leave the polenta to cool for about 20 minutes until cool and firm.
Preheat the oven to 220 degrees C. Spread the pesto evenly over the polenta base. Thinly slice the mozzarella, then cut into strips and scatter over the polenta base.
Arrange the tomato halves on top, cut-side uppermost. Sprinkle with freshly ground black pepper, then scatter the pine nuts over the top.
Bake for 15–20 minutes until golden and bubbling. Serve hot or leave to cool to room temperature, sprinkled with fresh basil leaves and some olives.