Fry the onions in the olive oil until golden brown.
Add the chopped garlic and fry for a minute or so more. Add the passata or chopped tomatoes and parsley and simmer for about 20 minutes, or until the sauce has thickened.
Season to taste with salt and freshly ground pepper.
Instead of, or as well as, parsley, a good pinch of oregano can be added.
Preheat the oven to 250oC/ Gas Mark 9. Place the polenta and water in a large saucepan. Turn the hob to the highest setting and bring the mix to the boil, then immediately turn down to medium heat, stirring continuously until the polenta has thickened.
Turn the cooker down to the lowest setting, cover the saucepan and allow the thickened polenta to cook very slowly for about 10 minutes to ensure that it is softened and all the starch has been cooked.
Season with salt and pepper and, if desired, whisk in butter or olive oil for added richness.
Pour the polenta onto a greased tray on a circle shape, the size of the pizza that is desired, then allow it to set until firm, preferably in the fridge.
Bake in oven for 20 minutes, or until crisped on the top.
Top with the tomato sauce, cubed mozzarella, chopped parsley, dried oregano, olives and anchovies, if desired.
Return to the oven until the topping and cheese has melted and browned.