In a greased 3-quart rectangular baking dish combine potatoes and onions. In a small bowl combine water, oil, sage, 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. Season to taste with additional salt and pepper. Makes 8 servings.