Preheat oven to 350 degrees fahrenheit
Butter the bottom and sides of a large 9 by 12 baking dish.
With a mandolin, slice the potatoes ⅛th inch thick and stack vertically in the baking dish until filled.
In a medium sized skillet over medium low to medium heat, place 3 tablespoons of butter. Once melted add 3 tablespoons of flour and whisk together until there are no more lumps for about 1 minute.
Next, very slowly add in the heavy cream while still whisking constantly.
Then add the garlic powder, onion powder, paprika, salt and pepper still constantly whisking.
Next add the cheese, a small handful at a time whisking until it has dissolved in the cream sauce until all cheese has been incorporated.
Once all the cheese has melted and the sauce is smooth, carefully pour over the potatoes making sure each potato has sauce poured over it.
Cover with foil and bake for 1 hour. Then remove from the oven and discard the foil. With a fork, gently separate each potato slice, letting the sauce seep in between each slice.
Return to the oven, uncovered and continue baking for 30 more minutes until golden brown and bubbly. Let cool for at least 10 minutes and garnish with fresh parsley or thyme.