Bring the water to a boil in a large pot. 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 pot from the heat and stir in the corn, scallions, oregano and freshly ground black pepper to taste. Transfer the polenta mixture to an oiled Swiss roll pan or rectangular baking pan measuring about 30 x 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 425°F (220°C). Spread the pesto evenly over the polenta base. Thinly slice the mozzarella, then cut into strips and scatter over the polenta.
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.
Variation: Replace the sun-dried tomato pesto with basil pesto, and top with thin rings of red pepper and halved Roma tomatoes.
Per serving: 515 calories, 17.5 g fat (5.5 g saturated), 24 g protein, 61 g carbohydrate, 8 g fibre