Ingredients
Scale
- 4 lbs Russet or Yukon Gold potatoes, peeled and sliced into 1/8-inch rounds
- 1/4 cup unsalted butter
- 1 medium yellow onion, finely diced
- 3 cloves garlic, minced
- 1/4 cup all-purpose flour
- 2 cups whole milk
- 1 cup heavy cream
- 1 teaspoon salt (plus more for potato layers)
- 1/2 teaspoon black pepper
- 1 teaspoon fresh thyme leaves
- 2 cups sharp cheddar cheese, shredded
- 1/2 cup Parmesan cheese, grated
Instructions
- Step 1: Sauté the Aromatics and Build the Roux: In a large saucepan or Dutch oven, melt the butter over medium heat. Add the diced onions and cook for about 5 minutes until they become translucent and soft. Stir in the minced garlic and cook for another 60 seconds until fragrant. Sprinkle the flour over the butter and onion mixture. Whisk constantly for 2 minutes to cook out the raw flour taste, creating a light golden paste.
- Step 2: Create the Creamy Sauce: Slowly pour in the whole milk and heavy cream while whisking vigorously to prevent lumps. Continue cooking over medium heat, stirring frequently, until the sauce thickens enough to coat the back of a spoon. Stir in the salt, pepper, and fresh thyme. Remove the pan from the heat and stir in 1 cup of the shredded cheddar cheese until melted and smooth.
- Step 3: Layer the Potatoes: Grease a 9×13-inch baking dish with butter or non-stick spray. Arrange one-third of the potato slices in an even layer at the bottom of the dish. Pour one-third of the cheese sauce over the potatoes, spreading it to the edges. Repeat the layers twice more with the remaining potatoes and sauce. Top the final layer with the remaining cup of cheddar cheese and the Parmesan cheese.
- Step 4: The Make-Ahead Chill or Immediate Bake: If making ahead, let the casserole cool to room temperature. Cover tightly with plastic wrap followed by aluminum foil and refrigerate for up to 48 hours. If baking immediately, preheat your oven to 375°F (190°C). Cover the dish with foil and bake for 45 minutes. Remove the foil and bake for an additional 25 to 30 minutes until the potatoes are fork-tender and the top is bubbly and golden brown.
- Cook Time: 1 hour 45 minutes