Preheat your oven to 375°F (190°C).
Peel the russet potatoes and slice them thinly, about ⅛ inch thick.
In a mixing bowl, combine the heavy cream, minced garlic, thyme, salt, and black pepper. Whisk until well mixed.
Grease the 9x13 inch baking dish with butter.
Layer half of the sliced potatoes in the bottom of the baking dish, slightly overlapping each slice.
Pour half of the cream mixture over the first layer of potatoes, ensuring even coverage.
Sprinkle half of the shredded cheddar cheese and half of the grated Parmesan cheese evenly over the cream layer.
Repeat the layering process with the remaining potatoes, cream mixture, and cheeses.
Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes.
After 45 minutes, remove the foil and increase the oven temperature to 400°F (200°C). Bake for an additional 15 minutes, or until the top is golden and bubbly.
Remove from the oven and let it rest for 10 minutes before serving.