Scrub the potatoes with a vegetable brush under cold running water.
Peel. Cut into thin slices. Place in a bowl of cold water and set aside.
Wash leeks well, trim off green ends and roots and cut into thin slices. Set aside.
Rinse asparagus and trim off woody ends.
In large pot over medium heat, heat chicken broth and water to boiling. Add asparagus and cook until bright green and still tender, about 2 to 3 minutes. Drain asparagus, reserving broth, and place asparagus in bowl of ice water to stop the cooking.
When cool, cut of tips, wrap in plastic wrap and refrigerate for garnish. Cut remaining asparagus into 1 inch pieces and set aside.
In same pot, over medium heat, melt butter. Add leek slices and onion and cook until beginning to get tender, about 5 minutes. Drain potato slices and add to onions in pot.
Add reserved broth and thyme spring. Over medium heat, heat to simmering. Cover and cook until potatoes are tender, about 10 to 15 minutes. Remove from heat and let cool slightly.
In blender, blend the asparagus pieces with ½ cup cold water and 3 ice cubes until smooth. Pour into another container, cover and refrigerate overnight. Add potato mixture to blender and blend until smooth. Pour into another container, cover and refrigerate overnight.
Next day, combine the asparagus puree, potato mixture, milk and cream. Taste and add salt and white pepper as needed.
Serve in chilled soup bowls. Garnish with reserved asparagus tips, Lemon-Zest Crème Fraich and chive blossoms or chopped chives.
Lemon-Zest Creme Fraiche
Blend 1/3 cup Crème Fraiche with ½ teaspoon lemon zest.
Nutritional Analysis per serving: 203 calories, 6 g protein, 25 g carbohydrates, 9 g fat (39% of calories from fat), 28 mg cholesterol, 2 g fiber, 136 mg sodium.