Preheat the oven to 200C/190C fan/ Gas 6. Roughly chop the tomatoes and place on one side of a baking tray. Peel and chop the onion and add to the tray. Drop on the unpeeled garlic clove. Drizzle over the balsamic vinegar and 1tbsp olive oil. Tumble in the cherry tomatoes on to the other side of the tray.
Slide the tray into the oven and roast for 30 mins till the edges are charred. Roughly chop the sundried tomatoes and place in a bowl. Pour over 200 ml boiling water and leave to soak. Pour the balsamic vinegar over the tomatoes and put back into the oven for 10 mins.
Scoop the tomatoes and onions into a pan. Leave the cherry tomatoes on the tray. Squeeze the roasted garlic clove into the pan. Sprinkle in most of the thyme leaves. Pour in the sundried tomatoes and their soaking water. Pour in 400ml boiling water into the pan. Bring to a simmer.
Blend till smooth using a stick blender or transfer into a food processor. Place the soup back on a gentle heat and tip in the whole roasted cherry tomatoes. Taste and season. Serve in warm bowls with any remaining thyme leaves sprinkled on top.