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

Using a mandoline slicer or a sharp knife, thinly slice the russet potatoes, aiming for about 1/8-inch thickness for even cooking.

In a large mixing bowl, combine 1 cup of heavy cream, 1/2 cup shredded white cheddar cheese, 4 tablespoons grated Parmesan cheese, and the minced garlic. Stir this mixture until well blended and creamy.

Add the sliced potatoes to the cheese sauce, stirring gently but thoroughly to coat each piece evenly. Season generously with salt and pepper.

Layer the potato slices upright in the greased baking dish, stacking them neatly. Pour the remaining cheese mixture over the potatoes and dot the top with the remaining 2 tablespoons of butter.

Cover the dish tightly with aluminum foil and bake in the preheated oven for 35-40 minutes.

After the initial baking, remove the foil and sprinkle the remaining 2 tablespoons of grated Parmesan over the top. Bake uncovered for an additional 25-30 minutes, or until the potatoes are fork-tender and the top is golden brown.

For the final touch, set your oven to broil and toast the top of the dish for 1 minute, keeping a close watch to avoid burning. Let it sit for 10 minutes before serving.