Heat the oven to 200C/Fan 190C/Gas 6. Roughly chop the tomatoes and place them on one side of a baking tray. Peel and chop the onion and add to the middle of the tray. Drop on the unpeeled garlic clove. Tumble the cherry tomatoes on to the other side of the tray. Drizzle over 1 tbsp olive oil and season with a pinch of salt and pepper.
Slide the tray into the oven and roast for 30 mins till the edges of the tomatoes are charred. While the tomatoes roast, roughly chop the sun dried tomatoes and place in a bowl. Pour over 600ml boiling water and leave to soak.
Take the baking tray out of the oven. Scoop out the red onion and garlic clove and set to one side. Pour the balsamic vinegar over the tomatoes and roast for 10 more mins.
After 10 mins, scoop the chopped tomatoes and onions into a pan. Leave the cherry tomatoes on the tray. Squeeze the roast garlic clove into the pan. Sprinkle in most of the thyme leaves. Pour in the sun dried tomatoes and their soaking water. Bring to a simmer.
Blend till smooth using a hand-held blender or transfer to a blender and blitz. Place the soup back on a gentle heat and tip in the whole roast cherry tomatoes. Warm through for 1-2 mins. Taste and season. Serve in warm bowls with the remaining thyme leaves sprinkled on top.