Pour 400ml water into a large saucepan and bring to the boil. Add the polenta gradually, stirring all the time. Cook, stirring frequently, over a medium-low heat for 4-5 minutes for a firmer texture. Add the parmesan and stir through.
Preheat the oven to 200°C/fan 180°C/gas mark 6. Spread the polenta on to a lined, circular baking tray that's around 25cm in diameter. Leave to cool, then bake in the oven for 15 minutes.
Meanwhile, heat the olive oil in a pan and add the garlic. Cook for 1 minute before adding the spinach. Coat with the garlic and cook for 2-3 minutes until just wilted. Squeeze out any excess moisture.
Remove the polenta base from the oven and top with the spinach, then scatter over the mozzarella. Put back in the oven and cook for 18-20 minutes. To serve, add roughly torn strips of the parma ham and cut the pizza into slices.