1 Medium Yellow Onion small diced
2 Garlic Cloves minced
3 T Butter, unsalted
3 T All Purpose Flour
3 c Heavy Cream or Half And Half
1 t Dried Thyme
1 t Sea Salt, fine
1 t Fresh Cracked Black Pepper
1 1/2 c Sharp Cheddar Cheese, shredded
1 c Parmesan Cheese, grated
1. Preheat your oven to 400 degrees with the rack in the middle of your oven.
2. Peel and thinly slice your Potatoes paper thin. As thick as a thick Potato Chip. I use a mandolin to slice mine thin but a large knife works well.Using my mandolin. The best ones are from Japan or France. Use caution! A mandolin is very sharp! Be smarter than me and use the guard.
3. Butter a large deep casserole dish all over well with 2 T of Butter.
4. Dice the Onion into small dice. Mince the Garlic fine.
5. Saute over medium heat the Onion in 3 T of Butter until soft. Add the Garlic and saute for 1 minute more.6. Add 3 T of Flour and stir well. Cook for 2 minutes so the flour cooks.
7. Add 3 c Heavy Cream or Half And Half, dried Thyme, Sea Salt and Pepper and mix in well. Turn up the heat to medium high and bring to a boil constantly stirring. Boil for 1 minute and reduce the heat to medium and simmer 4-5 minutes constantly stirring.
8. The Sauce should be thick and coat the back of a spoon. Remove the sauce pan from the heat.
9. Add all the Cheddar Cheese and 1/2 c of the Parmesan Cheese off heat and stir in well. The Sauce will thicken. Taste the Sauce and adjust for Salt and Pepper if needed.
10. Layer your Casserole dish with the thin layers of Potatoes over lapping as you go.
11. Layer the Cheese Sauce between layers of sliced Potatoes ending with a layer of Cheese Sauce on top.
12. Sprinkle the top with the remaining 1/2 c of Parmesan Cheese. Cover with aluminum foil and place the dish on a cookie sheet to make transfer easier and safer to and from the oven. Bake at 400 degrees for 1 hr 45 minutes covered. Remove the foil and continue baking for 15 minutes more so the top will begin to brown. Keep your casserole in the middle of the oven and turn your broiler on HIGH and broil the top until it is bubbling and well browned in spots for the best flavor. Watch it close because the broiler can quickly cause it to burn. It may take 5 minutes or so to broil the top well. Remove from the oven and allow to sit for 15 minutes so the Sauce sets and you don't burn your tongue! It's very hot right from the oven.