Peel and slice the Yukon gold potatoes 1/8-inch (3mm) thick using a mandoline. Soak in cold water, then pat completely dry. Dice the ham into 1/2-inch cubes.

Melt the unsalted butter in a heavy-bottomed saucepan over medium heat. Sauté the diced yellow onion for 3-4 minutes, add the garlic for 30 seconds, then whisk in the flour and cook for 1-2 minutes until a blonde paste forms.

Gradually whisk in the whole milk and heavy cream to prevent lumps. Simmer for 3-5 minutes until the sauce coats the back of a spoon. Stir in the salt, black pepper, and nutmeg.

Preheat oven to 375°F (190°C). Grease a 9x13-inch ceramic baking dish. Arrange one-third of the potato slices in an overlapping pattern. Top with half the diced ham, 1 cup of cheddar cheese, and one-third of the cream sauce. Repeat this layer once more.

Finish with a final neat layer of the remaining potatoes and top with the last of the cream sauce. Cover tightly with aluminum foil and bake for 45 minutes until the potatoes begin to soften.

Remove the foil, sprinkle the remaining 1 cup of cheddar evenly across the top, and bake uncovered for 25-30 minutes. Broil for the final 2 minutes to achieve a golden-brown, blistered cheese crust.

Remove from the oven and let rest for at least 15 minutes to allow the sauce to thicken. Garnish with finely chopped fresh green parsley before serving.