Step 1Peel the onion and cut it in half lengthwise. Cut off the ends, then cut the onion crosswise into one-eighth inch thick slices. Heat a medium frying pan and add the oil. Add the onion and cook until the onion is caramelized (soft and brown in color), tossing occasionally, about 25 to 30 minutes. Remove from the pan and cool slightly.
Step 2Grate the potatoes on the medium hole of a box grater. Combine the grated potatoes with the egg whites, salt, 4 ounces of goat cheese and the caramelized onions.
Step 3Using a medium-large non-stick skillet, heat one-half cup of the butter over low heat. Drop enough batter into the pan (about one-fourth cup) so that when flattened with the back of a spoon, the pancake is about 3 1/2 to 4 inches in diameter. Fry about 4 to 5 minutes on each side, turning when bottom turns golden brown. Cook in batches if necessary. Add more butter to the pan when necessary. Drain the pancakes on paper towels. To serve, crumble the remaining goat cheese and sprinkle a bit on each pancake.