Preheat the oven to 200C/Gas 6. Oil and line a small baking tray with nonstick baking paper. Heat the milk, stock and some seasoning in a large saucepan until almost boiling.
Pour in the polenta gradually, whisking continuously to prevent large lumps forming. Simmer and stir for about 5 minutes. Remove from the heat and stir in the butter and two thirds of the parmesan cheese. Taste and adjust the seasoning.
Spoon the polenta mix evenly into fishcake size shapes across the prepared baking tray (2 per serving). Arrange the finely diced tomatoes on the top of each cake. Sprinkle with the extra polenta and the remaining parmesan and bake for about 20 minutes, until the surface is golden. Serve in a bowl on the chopped tomatoes.