(From Dorie Greenspan's "Waffles From Morning to Midnight" (Morrow). Makes about 6 6-1/2" waffles. Peel, wash and cut the potatoes into uniform small pieces. Put in a large pot with cold water to cover, salt well, and bring to a boil. Then lower heat and cook until potatoes can be pierced easily with a fork; drain, reserving about 1/2 cup of potato water. Transfer potatoes to a large mixing bowl. While potatoes cook, heat olive oil and chopped onion in a small skillet over low heat just until onion softens a bit. Pour oil and onion over drained potatoes. Add milk to still-warm skillet just to take chill off then pour milk over potatoes. Using a fork, a potato masher or a rubber spatula, mash potatoes with oil and milk. Add 1/4 cup of warm potato water, reserving rest, and continue to mash until mixture is smooth. Taste potatoes and season liberally with salt and freshly ground pepper. Preheat waffle iron and finish batter by beating eggs into potatoes with a whisk or handheld mixer. Whisk together flour and baking powder and fold into potato mixture with a rubber spatula. Lightly butter or spray waffle iron grids. (You can skip this step with well-seasoned or non-stick materials.) Scrape batter by half-cup measure onto hot iron, smoothing batter evenly almost to edge of grids. Close iron and bake until brown and crisp.