Heat your oven to 180°C/Fan 160°C/Gas 4. Peel the onions and slice them into rings around ½ cm thick. Add to a roasting tin with 2 tbsp oil. Season well and turn to coat everything in the seasoning. Add most of thyme sprigs to the tin. Roast for 35 mins.
Stir 2 tbsp balsamic vinegar through the onions. Tuck the garlic bulb into the tin. Roast for another 20 mins till the onions are golden brown and the garlic is soft. Cover the sundried tomatoes with boiling water. Leave to soak till soft.
Crumble the stock cube into a jug. Add 750ml boiling water and stir to dissolve it. Lift the thyme sprigs out of the tin. Pick out the garlic and squish the cloves from the skins with a fork. Stir the garlic back into the onions with the stock.
Taste the soup and season if needed. Drain the tomatoes and slice them into strips. Ladle the soup into bowls. Top with the tomatoes and remaining thyme leaves. Serve straight away.