Grease a 9x13 casserole dish with butter and set aside.
Use a sharp knife or potato peeler to peel the skin off the potatoes.
Slice potatoes thinly using a knife or mandoline.
Place the stick of butter into a large skillet and melt butter over medium-high heat.
Add the flour and stir well until it's completely incorporated and smooth.
Add the heavy cream and whisk until smooth.
Add the Garlic Salt, and the dash of Tabasco Sauce, and stir well.
Bring sauce to a boil, and turn heat to low, stirring constantly for 1 minute.
Stir in the shredded cheeses reserving about 1/4 cup of each to sprinkle on top of dish.
Place half of potatoes in an even layer in the dish and spoon about half the sauce over the potatoes.
Place the remaining potatoes in the dish and spoon the remainder of the sauce over the second layer of potatoes.
Sprinkle the shredded cheese over the top layer of potatoes and place the dish in the oven.
Bake at 350 for about an hour. Keep watch on the potatoes, making sure the cheese on top of the potatoes doesn't burn.
If the cheese gets too dark, cover loosely with aluminum foil.
Remove the dish when baked and cool slightly before serving.
Top with minced parsley.
Serve and Enjoy!