Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with 2 tablespoons of softened unsalted butter.

Thinly slice 3 pounds of russet potatoes using a mandolin or sharp knife for about 1/8-inch slices. Place them in cold water to prevent browning.

In a mixing bowl, combine 1 cup heavy cream, ½ cup shredded white cheddar cheese, 4 tablespoons grated Parmesan cheese, and 3 minced garlic cloves. Stir until well-blended.

Drain and pat sliced potatoes dry, then add them to the cream mixture. Toss until evenly coated.

Arrange coated potato slices vertically in the greased baking dish, overlapping slightly. Pour remaining cream mixture over the top and season lightly with salt and pepper.

Dot the top with remaining 2 tablespoons of butter and cover the dish tightly with foil. Bake for 35–40 minutes.

After 35–40 minutes, carefully remove the foil, sprinkle remaining 2 tablespoons grated Parmesan cheese over the top, and return to the oven for an additional 25–30 minutes until golden brown.

Switch to broil and cook for about 1 minute to achieve a bubbly, golden top. Let cool for 10 minutes before serving.