Clean mushrooms and trim stem ends. Place in food processor with lemon juice. Process until finely chopped. Over low heat, melt butter in large skillet and add minced onion, saute until soft. Add mushrooms and cook, stirring frequently, until liquid is completely evaporated (about 10-15 minutes).
Add cream, chicken broth and pepper. Bring to a boil. Reduce heat and simmer 20 minutes. Add dissolved cornstarch and simmer 10 minutes (or until thickened). Adjust seasoning and ladle into serving bowls. Garnish with several tablespoons of chopped parsley. 4-6 servings.