Puréed green beans and fresh basil combine to form a compelling and sophisticated soup.
In a saucepan over medium heat, melt butter. Add leeks and garlic and toss to coat. Cook until leeks are tender. Add chicken broth, potatoes, and green beans. Bring to a gentle boil. Cover pan and cook for 20 minutes, or until potatoes and green beans are tender. Stir in basil and cook, uncovered, for 5 minutes.
Pour contents of pan into a blender or food processor and process until smooth. Return to pan and stir. Add cream, savory, lemon juice, salt, and cayenne. Warm gently, but do not allow soup to boil. Ladle into bowls and garnish with mushrooms.