In large fry pan saute onion in the wine until the liquid is just about gone. Add butter and let it melt. Add mushrooms and cook about 5 minutes. Add milk, flour paste and bouillon. Heat over moderate heat, stirring frequently until soup thickens and starts to boil. Remove from heat, add pepper to taste. Stir a little hot soup into the sour cream. Stir mixture back into soup. Serve at once. Garnish with fresh dill if desired. Yield: 4 servings.