1. Mix the yogurt and tarragon and set aside to let the flavor infuse.

2. Remove the tough ends from the asparagus. Reserve two heads for garnish for every bowl you plan to serve, and chop the rest into 1 to 2 inch pieces.

3. Peel and cube the potatoes.

4. Chop the onion and sautee in butter over medium high heat for 5 minutes or until translucent but not browned. Add the potato and sautee for two more minutes. Add the asparagus and sautee for about a minute more.

5. Add chicken broth and bring to a boil, then reduce heat and simmer for about 10 minutes, or until everything is tender. Put the reserved asparagus heads in a strainer and hang into the pot until bright green, then run under cold water and set aside.

6. Blend with a hand blender (or in a blender if you do not have one) until smooth. Return to heat, taste and season with salt and white pepper.

7. To serve, ladle some soup into a bowl. Put 1-2 teaspoons of the yogurt mixture in the center of the bowl, then top with two asparagus heads.