3poundsrusset potatoes,sliced 1/4 inch thick (use a mandolin for perfect and even slices) Place sliced potatoes into a large bowl of cold water. Let sit until ready to use.
1/4cupbutter,plus more for greasing casserole dish
1/2teaspoonground black pepper
4cupscheese,old white cheddar
1cupgrated parmesan cheese
2tablespoonsparsley,finely chopped, for garnish - optional
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 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 colour.
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 - whisk to incorporate until melted.
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 cup of 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 - if using, and let the potatoes set for 5 minutes before serving.