Preheat oven to 350 degrees. Lightly spray a 9x13 casserole dish with butter spray. Set aside.
In a bowl, combine diced onion, Parmesan Reggiano cheese, flour, parsley, thyme, garlic powder, Kosher salt, black pepper, paprika, and dry mustard. Stir to combine. Set aside.
Place half of the sliced potato mixture into the casserole dish. Sprinkle half the butter pieces over the potatoes, then half the onion Parmesan Reggiano mixture on top of the potato layer. Sprinkle half the shredded cheddar cheese mixture onto the potatoes.
Repeat the layering of potatoes, remaining butter, onion mix, cheddar cheese mixture.
Gently pour the cream over the entire mixture.
Cover with aluminum foil and place in oven to bake for an hour. Remove cover and place back in oven for about 15 minutes. (Note 5)
Serve and enjoy.