Tip: (after slicing potatoes, place in a large bowl of boiling hot water to par cook them while making the roux. This will lessen the cook time in oven.) The thinner you cut the potatoes the faster the cook time also.
Preheat oven to 400 degrees. Butter the bottom and sides of a 9 x 13 baking dish and set a side.
In a large skillet over medium high heat melt the butter to a sizzle. Add flour and whisk into butter for one minute (to cook out the flour taste).
Gradually add in the milk and seasonings. bring to a boil and continue to stir until thickened. Turn off burner. Stir in 2 cups of cheese until melted.
Remove potatoes from hot water and blot dry. Add 1/3 potatoes to prepared baking dish. Top with 1/3 of the sauce evenly.
Top with another 1/3 layer of potatoes and 1/3 layer of sauce. End with the last 1/3 of potatoes and the rest of sauce.
Pour the heavy cream evenly over the top and sides of potatoes. Top with remaining 1 cup of shredded cheese.
Cover with foil and bake for 45 minutes to an hour depending on how thin the potatoes were cut.
Remove foil and place back in oven to cook an additional 15 - 20 until cheese is a light golden brown and the potatoes are fork tender. (May need additional time depending on thickness of potatoes and if they were par cooked first.
