Prepare the Cream Mixture

• In a large measuring cup or bowl, combine the melted butter, heavy cream, minced garlic, and half of the minced thyme leaves. Stir well to mix. Microwave on high for about 1 minute, just to scald the milk and infuse it with the garlic and thyme flavors. Set aside.
  1 ½ cups heavy cream, 3 cloves garlic, 4 tablespoons salted butter, 1 tablespoons fresh thyme leaves

Preheat the Oven

• Set the oven to 350°F (180°C).
• Grease a 9x13-inch baking dish with cooking spray.
• In a medium bowl, mix all the cheese together and set aside.
  2 cups Gruyère cheese, 1 cup sharp white cheddar cheese, ½ cup parmesan cheese

Slice the Potatoes

• Peel the Russet potatoes.
  2 ½ pounds Russet potatoes
• Using a mandolin slicer, slice them into ⅛-inch thick rounds for even cooking.

Assemble the Layers

First Layer:

• Spread ⅓ of the sliced potatoes evenly in the greased baking dish.
• Pour ⅓ of the cream mixture over the potatoes.
• Sprinkle with ⅓ of the salt, pepper, and thyme.
• Top with 1 cup of the cheese mixture.

Second and Third Layers:

• Repeat the layering process two more times, reserving the remaining cheese for later.

Bake Covered

• Cover the baking dish with foil or a lid.
• Bake for 1 hour 15 minutes to 1 hour 30 minutes, until the potatoes in the center are tender when pierced with a knife.

Add the Final Cheese Topping

• Remove the foil.
• Sprinkle the remaining cheese mixture over the top.
• Bake uncovered for 10 to 15 more minutes, until the cheese is golden and bubbly.

Rest & Serve

• Let the dish sit for 5 minutes before serving. This helps the layers set for easier scooping.