To toast the flour
Toast the flour in a pan over a medium heat, stirring constantly until it turns hazelnut brown, remove, allow to cool slightly. Reduce the heat, warm the butter in the same pan. Sauté the onion for approx. 3 mins. Pour in the wine and reduce completely. Return the flour to the pan, pour in the stock, bring to the boil while stirring with a whisk. Reduce the heat, simmer for approx. 1 hr. while stirring occasionally, season.
Serve the soup in warm dishes, sprinkle with the Gruyère or serve separately.