Preheat the oven to 350°F. Melt the butter in a saucepan set over medium heat.
4 tablespoons unsalted butter
Add in the onion and cook for a few minutes until softened.
½ onion
Whisk in the flour and cook for a minute.
⅓ cup all-purpose flour
Slowly pour in the whole milk and chicken broth while whisking.
2½ cups whole milk, 1 cup low-sodium chicken broth
Let the mixture simmer until thickened a bit. Then, whisk in the salt, pepper, and Dijon mustard.
1 teaspoon kosher salt, ¼ teaspoon ground black pepper, 1 tablespoon Dijon mustard
Layer ⅓ of the potato slices in the bottom of a greased 9x13-inch casserole dish.
3½ pounds Yukon Gold potatoes
Top with 1 cup of diced ham and ⅓ of the sauce.
3 cups diced ham
Repeat twice more with a layer of potatoes, ham, and sauce.
Cover with aluminum foil and bake for 45 minutes until the potatoes are tender and the sauce is bubbling.
Remove from the oven, take off the foil, and add the cheese on top.
1½ cups freshly shredded cheddar cheese
Bake uncovered for another 10-15 minutes until the cheese is melted and bubbly.
Finish with a sprinkle of fresh chopped parsley.
Chopped fresh parsley