Peel potatoes and place them in ice water to prevent their browning until ready to grate.
Place egg yolks in a large bowl.
Lightly beat the yolks, stir in onions and flour mixed with salt and pepper.
Pat the potatoes dry.
Coarsely grate them onto a tea towel.
Squeeze potatoes in the towel to release unwanted moisture.
Add potatoes to egg-onion mixture and blend well.
Whip egg whites until stiff but not dry and fold into batter.
Melt 1 tablespoon margarine in a large heavy skillet over medium heat.
For each pancake, drop 1 heaping tablespoon of batter into the pan.
Flatten into thin, 3-inch wide pancakes.
Saute 2 to 3 minutes on a side, until golden brown and cooked through.
Add more margarine to the pan as necessary to complete cooking all the batter.
As they are done, place pancakes in a 200-degree oven to keep warm.