Peel potatoes and onion. (If not using immediately, place in cold water.) Grate with the shredding blade of food processor.
Put grated potatoes in a large strainer and press out liquid. Pour cold water over the potatoes and press out liquid once more. Transfer to large mixing bowl. Add onions.
Beat eggs and egg whites until thick. Add to potatoes and onions. Add flour or matzo meal, baking powder, and seasonings. Mix well.
Heat oil in frying pan. When ready, drop batter by large soup spoonfuls to form oval shapes. Fry over moderate to high heat until brown on one side; turn to brown other side. Remove from frying pan and drain on paper towels.