In a large heavy pot melt the butter over medium heat. Add the leeks, onion, garlic, and 2 tablespoons of water and cook approximately 10 minutes or until the leeks are golden brown.
Add potatoes and broth, bring to a boil. Reduce heat to low and simmer approximately 10 minutes or until potatoes are fork tender. Stir in tarragon, cream and yogurt, season to taste with salt and pepper.