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 the oil. Season well and turn to coat everything in the seasoning. Add most of thyme sprigs to the tin. Roast for 35 mins.
Stir the whole garlic cloves and 2 tbsp balsamic vinegar through the onions. Roast for another 15-20 mins till the onions are golden brown and the garlic is soft. Cover the sun dried tomatoes with boiling water. Leave to soak till soft.
Crumble the stock cube into a jug. Add 750 ml boiling water and stir to dissolve it. Lift the thyme sprigs out of the tin. Pick out the garlic cloves and squish the flesh 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. Drizzle over more balsamic vinegar to taste.