1. Heat oil in a large saucepan over medium-low heat. Add leeks and cook, stirring often, until softened but not browned, about 5 minutes. Add garlic and cook, stirring, for 1 minute.
2. Add potatoes and broth; bring to a simmer over medium-high heat. Cover and reduce heat to medium-low. Simmer, stirring occasionally, until the potatoes are tender, 10 to 15 minutes.
3. Increase heat to medium-high and stir in asparagus and peas; simmer, covered, stirring 2 or 3 times, until just tender, 3 to 4 minutes. Remove from heat; stir in 1 tablespoon chives, parsley, dill and chopped chervil (or parsley). Transfer the soup to a blender and blend until smooth. (Use caution when pureeing hot liquids.)
4. Return the soup to the pan. Add milk and bring to just below a simmer, stirring, over medium heat. Stir in lemon juice, salt and pepper. Ladle into soup bowls. Garnish each serving with a dollop of yogurt, a sprinkling of the remaining chopped chives and a sprig of chervil (or parsley).

Yield: 6 servings