Heat the butter in a pan. Add the onions, sweat for approx. 20 mins., stirring occasionally until the onions turn brown and soft. Add the flour and cook briefly.
Pour in the stock and wine, add the thyme and bay leaf, bring to the boil. Cover and simmer over a medium heat for approx. 20 mins. Add the vinegar, season.
Place the slices of bread on a baking tray lined with baking paper, top with the cheese and thyme.
Approx. 10 mins. in the centre of an oven preheated to 220°C. Plate up the soup, top with the cheesy bread.