Line a 20cm square baking tin with cling film. Bring the stock to the boil in a large pan. Pour the polenta into the stock in a steady stream, stirring all the time. Keep stirring and cook for 5 mins, then add the parmesan and chives. Season. Transfer the polenta to the prepared tin, let cool and then put in the fridge to chill for 1 hour.
Heat the oven to 180°C, fan 160°C, gas 4. Arrange the tomatoes (on their vines) and the mushrooms on a baking tray. Drizzle with the balsamic vinegar and 2 tbsp of the oil. Season. Roast for 10 mins until the tomatoes are starting to split and the mushrooms are cooked.
While the vegetables are roasting, heat a griddle pan over a high heat. Turn the polenta out of the baking tin, cut into 4 squares and brush all over with the remaining oil. Griddle for 3 mins each side.
Arrange the polenta on plates and top with the roasted tomatoes, mushrooms and a drizzle of the warm pan juices. Crumble over the vegetarian Roquefort, if using, and garnish with the remaining chives.
Cook’s tip: you can prepare the polenta in advance – it will keep in the fridge for a couple of days. For a smooth mixture, use a whisk until the liquid and polenta are combined and then change to a wooden spoon.