In a greased 3-quart casserole, combine potatoes and onions. In a 2-cup measure combine water, oil, sage, salt, and pepper; drizzle over potatoes and onions.

Bake, uncovered, in a 450 degree F. oven for 40 to 50 minutes or until the vegetables are tender, stirring twice.

Transfer to a medium mixing bowl. Mash with a potato masher or beat with an electric mixer on low speed. Gradually beat in enough buttermilk, yogurt, sour cream, or milk to make light and fluffy. Makes 10 side-dish servings.