Melt the butter in a saucepan and gently sauté the onions until soft and translucent.
In a heatproof jug, mix the wines and 1.5 litres boiling water. Sprinkle the onions with flour and stir, then pour the liquid over the onions and stir again.
Simmer gently for 15-20 minutes, skimming any impurities from the surface. Add a little more boiling water if your soup is looking too thick. Add the sugar and season to taste.
To make the croutons, rub one side of the baguette slices with olive oil and sprinkle with grated cheese. Grill until bubbling. Pour the soup into bowls, float the croutons on top and enjoy.
Recipe from Fiona Gibson, the author of The Woman Who Upped and Left, a heartwarming novel about a woman with a taste for adventure… and mouthwatering French food. Available now in paperback and on Amazon.
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