Grandmacore Scalloped Potatoes

Preheat your oven to 400°F (200°C). Grease a 9x13 inch baking dish with a little butter. Wash, peel (optional for rustic look), and thinly slice 3 lbs of Yukon Gold potatoes to about 1/8-inch thickness using a mandoline or sharp knife. Finely mince 1 small yellow onion and 3 cloves of garlic.

In a medium saucepan over medium heat, melt 4 tbsp of unsalted butter. Add the minced onion and sauté for 3-4 minutes until softened. Add the garlic and cook for another minute until fragrant.

Sprinkle 1/4 cup of all-purpose flour over the onions and stir constantly for 1-2 minutes to cook out the raw flour taste, creating a blonde roux.

Gradually whisk in 2 cups of whole milk and 1 cup of heavy cream, ensuring there are no lumps. Bring to a gentle simmer and cook until slightly thickened, about 3-4 minutes. Stir in 1 tbsp fresh thyme leaves, 1/4 tsp ground nutmeg, 1 1/2 tsp salt, and 1/2 tsp black pepper. Remove from heat.

Grate the cheeses to yield approximately 2 cups of Sharp Cheddar (8oz block) and 1 cup of Gruyère (4oz block). Mix them together in a bowl.

Assemble the casserole: Layer half of the sliced potatoes in the prepared baking dish. Pour half of the cream sauce over the potatoes and sprinkle with half of the cheese mixture. Repeat with the remaining potatoes, remaining sauce, and top with the remaining cheese.

Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 40-45 minutes, or until the potatoes are tender when pierced with a knife and the cheese is golden brown and bubbly.

Remove from the oven and let the casserole rest for at least 15 minutes before serving to allow the sauce to set.