Preheat oven to 400°F.
Wash and dry the potatoes. Pierce a few times with a knife to allow steam to excape while they bake in the oven.
Bake potatoes for about 1 hour and 15 minutes, or until potatoes are soft.
Allow them to cool to room temperature and refrigerate overnight.
Preheat oven to 425°F.
Generously butter an oven safe casserole dish.
Grate potatoes, using the large holes on a cheese grater, skin and all, into a large mixing bowl.
Add finely minced onion, salt, pepper and cayenne pepper. Toss together gently with two forks.
Scoop from underneath and mix together all the ingredients.
Add the shredded cheddar cheese.
Use forks to toss grated potatoes with cheese. The goal is to keep everything light and fluffy.
Add sour cream in blobs all along the top and use the "2 fork toss method" to combine the sour cream with the potato/cheese mixture. Be as gentle as you can while tossing together ingredients.
The goal is to keep the mixturre light and fluffy, not compacted.
Use forks to transfer mixture into the buttered casserole dish, being careful not to compact or compress the mixture together. Pile mixture up as high as you can and gently spread mixture out with the tips of the forks.
Top with a sprinkle of Parmesan cheese.
Bake for 30 minutes or until heated through and lightly golden brown.