Put the stock in a saucepan and bring to the boil, then stir in the polenta and Parmesan over a low heat. Continue to cook, stirring, until thick.
Turn out on to a lightly oiled surface and spread out to a thickness of 1 cm (½ inch). Using a plain cutter, stamp out 10 rounds a little larger than the tomato slices and place on a lightly oiled baking sheet.
Top each 'pizza' with a slice of tomato, some ham, Gorgonzola and pine nuts and drizzle over a little oil.
Place in a preheated oven, 220°C (425°F), Gas Mark 7, for 10–12 minutes until golden. Scatter over the basil leaves and serve.