Grate onion in blender just as you did the potatoes. Mix up batter, add salt and white pepper. Add matzo meal or flour, mix well and fry in hot oil. The batter should be like semi-thick oatmeal. You may have to add more flour and matzo meal as you cook.
Batter tends to get more liquidy as it stands.
These can be frozen on a cookie sheet, placed loose pack when frozen in plastic bags. Reheat frozen latkes in 425 oven until hot. For good results, use only high gluten flour from a flour mill. Commercial bread flour, e.g. Pillsbury or Gold Medal does not work as well. In the Central New York area, I use New Hope Mills or Mercer Milling flour.