Heat your oven to 200°C/Fan 180°C/Gas 6. Set a large roasting dish on the top shelf to warm. Finely chop the leaves from 1 rosemary sprig. Peel and finely chop 2 garlic cloves. Tip the rosemary and garlic into a pan with the polenta and a big pinch of salt and pepper. Whisk in 250 ml boiling water till thick and smooth. Pop in the fridge to set.
Quarter your tomatoes. Cut the peppers into 3-4 cm chunks, removing the seeds and stems. Tumble into the warmed dish with 1 tbsp oil. Season. Roast for 20 mins.
Halve, peel and chop your onion. Peel and chop the remaining garlic. Mix the onion, garlic and 2 tbsp balsamic in with the tomatoes. Roast for a further 5 mins.
Scoop the roasted veg into a blender or food processor. Blitz, trickling in the remaining boiling water, till it’s as thick or as thin as you like. Adjust seasoning to taste.
Gently warm the soup back up. Dollop in little spoonfuls of the polenta mix to the soup, creating little dumplings.