Bring 1½ litres water to the boil and add the stock pots and ½ tsp salt. Pour in the polenta, whisking continuously, bring to the boil and reduce the heat to low, then stir every minute to keep it from sticking, for about 8 mins (or according to pack instructions).
Meanwhile, oil a non-stick baking tray (ours was 30 x 40cm). Spoon the cooked polenta into the tray and level with a spoon to make a large, flat slab that fills the whole tray (about 1cm thick). You can use wet palms to press it down firmly after a few minutes. Leave to cool and set firm for at least 1 hr.
Turn out the polenta onto your work surface and cut into strips. Halve the strips on the diagonal to make little wedges and put back on the tray. Heat the oven to 180C/160C fan/gas 4.
Top each polenta base with 1 tsp pizza sauce, a little vegan cheese and a sprinkle of oregano. Add extra toppings, if you like. Drizzle with olive oil and bake for 20-25 mins. Serve warm or cold.