Clean leeks. Slice into saucepan along with onions and butter. Cover and simmer 30 minutes. Add mushrooms and cook 10 minutes. Stir in flour. Cook a few minutes.
Add stock, half-and-half cream, salt, sugar, and pepper. Stir-cook until slightly thickened. Grind through a food mill. Pour back into saucepan and keep warm. Stir in crème or whipping cream when ready to serve. Heat and then pour into a tureen. Garnish with sliced raw mushrooms and some minced parsley.