Preheat oven to 425 degrees. Line a baking pan or cooking sheet with foil. Coat a 9 x 9-inch pan (glass or metal) with cooking spray.
Place onions on a microwave-safe plate and cook for about 4 minutes in the microwave, until soft.
In a food processor or blender, add cooked onions, cottage cheese, buttermilk, and egg white. Process until smooth.
Peel all potatoes. Slice each very thin by hand or use a mandolin. Arrange ½ of the slices in pan. Start at the edges and work in, overlapping. Press down on potatoes then spread with ½ of cheese mixture. Season with a little salt, pepper, and 2 tablespoons of Parmesan cheese. Layer remaining potatoes and spread the top with remaining cheese sauce. Sprinkle with a little more salt, pepper, and top with 3 tablespoons Parmesan cheese. Cover with foil by making it a little tented so the foil does not touch the top of the casserole. You don’t want the foil to stick to the top of the cheese sauce.
Bake covered for 1 hour 20 minutes. Uncover and bake for about 30 minutes until very golden brown and potatoes are soft. You can stick a knife into the casserole to see if the potatoes are fully cooked. If not, cover again with foil and bake for an additional 15-20 minutes. Note: if you haven’t cut the potato slices thin enough, they could take longer to cook.
Remove from oven and let rest for at least 15 minutes before cutting into slices. This can be made the day before and reheated before serving.