Start by peeling your large Russet potatoes very well. Use a mandoline to slice them into thin rounds. Aim for slices that are about one-eighth inch thick. Keep them in cold water so they do not brown.

Melt the butter in your saucepan over medium heat. Add the diced onions and minced garlic to the pan. Cook them for three minutes until they look soft. Do not let the garlic turn dark or bitter.

Sprinkle the flour over the buttery onions and garlic. Stir constantly for one minute with your small whisk. This cooks out the raw flour taste very well. The mix should look like a thick golden paste.

Slowly pour the heavy cream into the warm saucepan. Whisk continuously to avoid any small flour clumps forming. Let it simmer until the sauce thickens like gravy. Remove the pan from the heat once it bubbles.

Add half of the shredded cheddar cheese to the sauce. Stir until the cheese is fully melted and smooth. Season the sauce with salt, pepper, and fresh thyme. This is the liquid gold for your potatoes.

Grease your baking dish with a little bit of butter. Arrange one-third of the potato slices in the bottom. Pour one-third of the cheese sauce over the top. Repeat these layers until you use all the potatoes.

Cover the dish with foil and bake at 375 degrees. Cook for forty minutes until the potatoes are very soft. Remove the foil and add the remaining cheese on top. Bake for fifteen more minutes until golden and bubbly.