This soup is a specialty from the French region of Franche-Comté. If you cannot find dried morels, which can be pricey, used a second packet of mixed mushrooms instead. The soup will still taste great, as dried mushrooms give it exceptional flavour.
Wash dried mushrooms well under cold water to remove sand. Soak in milk and water so that the mushrooms are almost fully immersed. Add cognac if liked.
In an large saucepan, melt butter over medium heat and add flour. Stir till a roux forms. Strain mushrooms reserving the liquid. Gradually add the liquid to the roux, whisking till it thickens. Add cream, salt and pepper. Set to one side.
In a frying pan melt butter and fry mushrooms over very low heat till soft, about 15 minutes, stirring often. Add mushrooms to the soup and reheat thoroughly. Just before serving, add mustard. Served with slices of toasted bread.