Line a shallow baking tray with baking parchment. In a large pot, bring 1 litre of water to the boil with 2 teaspoons of salt. Turn the heat to medium and whisk in the polenta. It will begin to thicken, at which point change the whisk for a spoon and cook over a moderate heat for 2 minutes, stirring constantly, but being careful that it doesn’t ‘plop’ on to your hand. Mix in 2 tablespoons of the olive oil, then tip out on to the lined tray and form into a rough 25cm square. Leave to cool.
Once the polenta has cooled, preheat the oven to 180°C/gas 4. Heat a pot and add 3 tablespoons of the olive oil with the onions and garlic. Sauté over a moderate heat, stirring frequently, to soften the onion. Add the red peppers and aubergine, and continue to cook for another 2 minutes, stirring frequently. Mix in the tomatoes, half the oregano and 4 tablespoons of water. Cover and cook for 7 minutes, stirring twice. Season, then continue to cook until the liquid has evaporated a little.
Once the polenta has cooled, cut it into 6 even-sized pieces and place back on the lined tray. Divide the vegetable mixture between the pieces of polenta and lay the mozzarella on top. Bake in the oven until the mozzarella begins to melt and colour, about 15 minutes. While it’s cooking, using a pestle and mortar, pound the remaining oregano and olive oil with ½ teaspoon of salt and the chives to produce a luscious green oil.