Potato Leek Soup

Ingredients 3 Tbsp. olive oil or butter 2 leeks sliced in ¼ inch rings 1 small onion, chopped 4 cloves garlic 2 Tbsp. water ½ lb red skinned potatoes, unpeeled, cut into ½ chunks 4 cups chicken broth 2 tsp. fresh tarragon ½ cup whipping cream ½ cup plain whole yogurt...