3 ½ pounds Yukon Gold potatoes sliced ¼ inch thick
2 1/4 cups milk
5 tablespoons butter
2 garlic cloves minced
1 ½ tablespoon Dijon mustard
¼ teaspoon thyme
2½ cups Gruyere
2 ½ cups cooked ham
Pre-heat oven to 375 degrees
Place potatoes in pot and add water to cover. Cover pot and bring to a boil. Uncover and boil potatoes for 3 to 4 minutes. Drain, recover pot and let potatoes sit for another 3-4 minutes, then set aside.
Heat milk until it simmers.
Place 4 tablespoons butter in saucepan under low heat and cook until foam, add garlic, cook 1 minute and then add the flour and cook for 2-3 minutes stirring constantly. Do not allow the roux to turn brown. Off heat add about ½ cup milk stirring until thick. Continue stirring while gradually adding the balance of the milk. Return to saucepan, bring to a simmer and continue to stir for about 5 minutes until the mixture thickens. Stir in the mustard and thyme
Set aside 1-cup sauce and 1/3 cup cheese.
Place a thin layer of the sauce on the bottom of 8x12 baking dish
Layer lasagna style 1/3 of the potatoes, then the ham and finally 1/4 of the cheese. Repeat the process two more times ending with the ham on top.
Spread the reserve sauce and cheese over the top, dot with butter and place in the oven.
Bake until bubbling, about 45 minutes.
The casserole can be refrigerated at this time or served immediately.