Set 1 cup frozen peas aside
Heat oil in saucepan over medium high heat. add shallots and saute until golden and almost tender. add remaining peas, 4 cups broth and 2 tablespoons tarragon. bring to boil, reduce heat to medium and boil until flavors blend and peas are tender. Cool slightly and place in blender. Blend until completely smooth. Thin with additional stock if necessary. Stir in remaining tarragon and peas. Heat through and serve with dollop of yogurt