Preheat oven to 450 degrees. Cover a baking pan with foil. Line the inside of a 9-inch springform pan with foil too. Spray the foil-lined pan 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 process until smooth. Pour enough of mixture to coat the bottom of the spring-form pan.
Peel all potatoes. Slice each very thin by hand or use a mandolin. Arrange 2 layers of slices on the pan. Start at the edge and work in, overlapping slightly. Press down on potatoes then spread with about ⅓ of cheese mixture. Season with salt and pepper and 2 tablespoons of Parmesan cheese. Layer remaining potatoes and cheese 2 more times, ending with cheese mixture. Season with a little salt and pepper and top with remaining Parmesan cheese (It made be made to this point, covered and refrigerated up to 1 day ahead).
Cover pan with foil and place on foil-lined pan because it tends to leak while cooking. Bake until potatoes are tender, about 1¼ hours. Remove foil cover and continue baking until cheese is golden brown, about 30 minutes.
Remove from oven and let rest for at least 30 minutes before removing from pan. Un-mold and cut into 10 slices.