- 3 lbs. russet potatoes, peeled and sliced 1/4 inch thick (use a mandoline for perfect and even slices) Place sliced potatoes into a large bowl of cold water. Let sit until ready to use.
- 1/2 cup yellow onion, finely chopped
- 1/2 cup scallion, finely minced
- 2 cloves garlic, minced
- 1/4 cup butter, plus more for greasing casserole dish – TFD prefers Kerry Irish butter
- 1/4 cup all-purpose flour
- 1/2 tsp. Kosher salt
- 1/2 tsp. freshly-ground nutmeg
- 1/2 tsp. freshly-ground black pepper
- 1/2 tsp. Madras curry powder (optional, leave out if you so prefer, but TFD likes it)
- 3 cups whole milk combined with 1/3 cup Dijon mustard or if you prefer a bolder look and flavor, use The Hirshon Spicy Pub Mustard
- 1 cup finely-minced, mildly-flavored deli ham – preferably minced from a single thick slice
- 3 cups grated aged white cheddar combined with 1 cup grated parmesan and 1 cup grated gruyere
- 2 Tbsp. parsley or chervil (TFD prefers chervil), finely chopped, for garnish
- Preheat oven to 425 degrees. Liberally grease a large casserole dish with butter. Set aside.
- In a sauce pan, add the butter and onions and cook over medium heat for 5 minutes. The onions should be cooked through, but not browned.
- Add the garlic and ham and cook for 1 minute.
- Add the flour, salt, and pepper. Whisk into the butter and onion mixture. Constantly whisk to ‘cook’ the flour for 5 minutes. The mixture will turn a very light, golden color.
- Add the milk one cup at a time, whisking well between each addition, ensuring that there are no lumps.
- Next, add 3 cups of the cheese and the minced scallions – whisk to incorporate until melted. Add all remaining ingredients except potatoes and parsley/chervil and whisk. Remove cheese and milk mixture from the heat.
- Now, layer half of the potatoes into the casserole, evenly distributing the potatoes so that there’s an even layer.
- Ladle half of the sauce over the top. (I like to gently tap the casserole dish firmly onto a large cutting board. This forces the sauce down into the nooks and crannies.
- Layer the remaining potatoes into the casserole. Then, ladle the rest of the sauce on top. Again, tap the casserole gently to help settle the sauce.
- Top with the remaining cheese.
- Place in oven and bake for 1 hour. (It’s a good idea to pierce the scalloped potatoes with a sharp knife to see if the potatoes are cooked all the way through. In some cases, depending on the oven temperatures, firmness of the potato, or even the size of the casserole, you may need 10-15 minutes more baking time.)
- Remove from oven. Top with chopped parsley or chervil and let the potatoes set for 5 minutes before serving.