Step 1

Preheat the oven to 350 degrees F.

Step 2

Grease a deep 10x15-inch baking dish.

Step 3

In a large pot over medium heat, melt the butter.

Step 4

Add the onion and the extra salt to the melted butter and sauté until the onion is translucent, about 5 minutes.

Step 5

Add the flour to the onion and cook, stirring constantly, until the flour is blended in, about 2 minutes.

Step 6

Add the heavy cream to the onion mixture and whisk to combine.

Step 7

Add the milk to the onion mixture and whisk to combine. Cook, stirring constantly, until the mixture is just starting to simmer and thicken.

Step 8

Add 1 teaspoon of the salt, the black pepper, and the cayenne pepper to the sauce mixture and stir to combine.

Step 9

Add the nutmeg to the sauce mixture, then turn the heat off.

Step 10

Add the Worcestershire sauce and the thyme to the sauce mixture and stir to combine.

Step 11

Evenly spread 1/4 of the sauce out into the bottom of the prepared baking dish.

Step 12

Shingle 1/3 of the potatoes on top of the sauce layer and season with 1/3 of the remaining salt.

Step 13

Drip 1/3 of the remaining sauce mixture over the seasoned potatoes and sprinkle with 1/3 of the cheese.

Step 14

Top the cheese layer with 1/2 of the ham slices.

Step 15

Shingle 1/2 of the remaining potatoes on top of the ham slices and season with 1/2 of the remaining salt.

Step 16

Drip just less than 1/2 of the remaining sauce mixture over the potato layer and sprinkle the sauce with 1/2 of the remaining cheese.

Step 17

Layer the remaining ham on top of the casserole.

Step 18

Shingle the remaining potatoes on top of the ham layer and season with the remaining salt.

Step 19

Spread the remaining sauce over the casserole using a spatula, then sprinkle the sauce with the remaining cheese.

Step 20

Bake the casserole until it is browned, bubbling, and the potatoes are very tender, about 1 hour 30 minutes.

Step 21

Transfer the baking dish to a wire rack and let the casserole cool, about 15-20 minutes.

Step 22

Serve.