Serve this classic French soup chilled in the summer, hot in the winter.
1. Melt butter in large saucepan over medium heat. Add leeks, fennel, and garlic; cover, and reduce heat to medium-low. Cook 20 minutes, or until leeks and fennel are very soft, stirring occasionally.
2. Increase heat to medium-high. When leeks begin to sizzle, add wine, and cook 1 to 2 minutes. Stir in potatoes, thyme, and 5 cups water. Reduce heat to medium-low, cover, and simmer 45 minutes, or until potatoes are very soft. Remove thyme sprig, and purée with immersion blender or food processor until smooth. Stir in sour cream, if using, and cool. Season with salt and pepper, if desired, and garnish with chives.
To freeze and enjoy later: Measure cooled soup into two 1-qt. resealable containers; freeze. To eat later: thaw, and serve at room temperature, or heat gently in medium saucepan until hot.