Dissolve butter in a frying pan, add sugar and let it brown. Steam the onion rings and the garlic in it, sprinkle with vinegar.
Add the white wine and let it boil a little. Add the meat broth, simmer 1500 cmin on low heat. Season with tarragon, salt and pepper.
Pour the broth into ovenproof cups, add white bread cubes and a layer of cheese on top, bake in a heated oven at 180° Celsius until golden brown.
Put on the table immediately.