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.
Notes
Note 1: Use either Russet or Yukon Gold potatoes and slice potatoes ⅛ inch thick. If you do not have a Mandolin slicer, thinly slice potatoes after you peel them. Very important, if potatoes are thicker, you will need to increase baking time for the potatoes. Note 2: Fat-free half-and-half can be swapped out for the light cream.Note 3: I use a low-fat cheese when possible. Note 4: If you prefer a Gluten-Free dish, use almond flour.Note 5: Depending on how thick you sliced the potatoes, they may need more time. If they are sliced ⅛ inch, they should be cooked through. Note 6: If you want to make ahead, make the day before and bake for one hour covered with foil. When ready to finish, cook another 20 minutes covered.I am not a registered dietician, I do my best to provide the most accurate nutritional values. Any nutritional data I provide is an approximation. Actual dietary information can vary based on ingredients and portion sizes