Peel onions and cut them into rings. Peel garlic and chop finely. Wash the oregano, dab dry and remove the leaves. Wash rosemary, shake dry, pluck needles from twigs.
Heat the oil in a large pot. Fry the onions, garlic and rosemary in it. Deglaze with 1 litre of water. Bring to the boil, stir in broth. Cover and simmer for about 20 minutes. Add wine to the soup, season with salt, pepper, oregano and sugar.
Pour soup into 4 ovenproof soup bowls. Put one slice of bread in each and sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes.
Remove from the oven and serve immediately.