In a greased 3-quart rectangular baking dish combine potatoes and onions. In a small bowl combine water, oil, herbs, the 1/2 teaspoon salt, and 1/4 teaspoon pepper; drizzle over potatoes and onions.
Bake, uncovered, in a 450 degree F oven for 40 to 45 minutes or until the vegetables are tender and browned, stirring twice.
Transfer to a large mixing bowl. Mash with a potato masher or beat with an electric mixer on low speed. Gradually beat in enough buttermilk to make smooth and fluffy. If desired, drizzle with additional olive oil. Season to taste with additional salt and pepper. Makes 8 servings.