Preheat your oven to 400 degrees F. Grease the bottom and sides of a 9x13 inch casserole dish with a little butter, or use nonstick spray. Set aside.

Melt 1/4 cup butter over medium heat in a large pot. Thinly slice 1 onion and add to the butter. Saute for 4-6 minutes until the onion is translucent.

While it's cooking, add 1 tablespoon kosher salt (use less if all you have is table salt), 1/4 teaspoon black pepper, 1/4 teaspoon cayenne pepper (this adds flavor not heat), and 1/4 teaspoon dried thyme.

After sautéing for 4-6 minutes, add 5 cloves of smashed and minced garlic. Let cook for about 30 seconds, until fragrant.

Gradually sprinkle in 6 tablespoons of flour, adding about a tablespoon at a time and stirring in between. It will form a thick paste with all the onions. Cook the flour for about 30 seconds to cook out the "raw flour" taste.

Use a whisk to SLOWLY stir in 4 cups of cream. The process of adding the cream should take at least 2-3 minutes. Your burner should still be at medium heat. Add a little bit of cream, whisk it in completely, then add a little more. The goal is to form a smooth sauce; if you add the cream all at once, you will have flour lumps.

Once all of the cream is added, stir in 1/2 cup parmesan cheese, sprinkling it in a little at a time so that it melts nicely.

(If your potatoes are not prepped and ready, remove the sauce from heat and set aside.)

Peel 4 pounds of russet potatoes** and slice thinly into 1/8 inch rounds with a sharp knife, mandolin, or food processor. See photos. If you have a food processor, I highly recommend using it. The slicer attachment cuts them to exactly the 1/8 inch size you need, and it's so much faster. If there is any lag time in between when you slice the potatoes and when you add them to the sauce, place them in a large bowl of water to prevent browning. Drain well.

Add the sliced potatoes to the pot of sauce with the heat set to medium. Use a rubber spatula to stir the potatoes into the sauce, separating any slices that stick together to make sure they are all coated in sauce. Let the mixture come to a low bowl over medium heat, then reduce heat to medium low and cook for 10 minutes, stirring occasionally. Be gentle when stirring your potatoes; you don't want to break them all into pieces. A rubber spatula is better than a wooden spoon.

After the cream and potato mixture has bubbled for about 10 minutes, turn off the heat.

Pour or spoon half of the mixture into the greased 9x13 inch pan. Sprinkle the first layer with 1 and 1/2 cups sharp cheddar cheese.

Top the cheese with the rest of the potato and cream mixture and spread to the edges. Sprinkle the top with 1 and 1/2 cups cheddar cheese.

Bake in the oven at 400 degrees for about 40 minutes.

You will know the potatoes are done when the mixture is very bubbly, the cheese has started to brown, and a butter knife inserted into the potatoes slides through easily. If your cheese is browning but the potatoes are not tender, cover with foil and bake another 10 minutes.

Let stand for 10 minutes before serving.

Store leftovers covered in the fridge. Leftovers will keep in the fridge for about a week. Reheat in the microwave for lunches. Or if you have enough leftover that you want to serve it for dinner again, sprinkle a little extra cheese on top and bake at 400 for 20 minutes, to recreate that perfect crispy topping.