Potato waffles with rosemary-garlic oil

2                    Russet potatoes
                        Salt to taste
     1/4   c            Extra-virgin olive oil
   1       sm           Onion, peeled, fine chopped
     2/3   c            Milk
                        Freshly ground black pepper
                        -to taste
   2       lg           Eggs
   1       c            All-purpose flour
   2       ts           Double-acting baking powder

  (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.