Preheat oven to 400 degrees F.

Wash potatoes and thinly slice into ⅛-inch thick slices. Set aside.

Add ghee to a large skillet heated over medium-high. Once ghee is heated, add sliced onion and cook until soft, about 4 to 5 minutes.

Add minced garlic and ½ teaspoon kosher salt to the skillet and cook for another minute. Reduce heat to medium.

Stir in arrowroot flour and dried mustard. Continue stirring for about 30 to 45 seconds before slowly pouring in the almond milk, stopping to stir about halfway through. Add 1 teaspoon salt, ½ teaspoon pepper and 1 tablespoon thyme.

Continue stirring until sauce starts to thicken and just simmer. Remove from heat.

Arrange a layer of potato slices to cover the bottom of a 2½ or 3 quart baking dish. Add a few spoonfuls of the sauce and repeat layers two more times, finishing with the remaining sauce on the top layer. Use the back of a spoon to smooth sauce completely over the top layer.

Cover the baking dish with foil and bake in preheated oven for 30 minutes. Remove foil and bake for another 25 to 30 minutes, until potatoes are cooked through and top is golden brown.

Let rest for a few minutes and serve.