Begin by peeling your russet potatoes and slicing them thinly—about 1/8 inch thick works best. Using a mandoline slicer ensures consistency, which helps the potatoes cook evenly. Place the slices in a large bowl as you work.
In a small saucepan, melt the unsalted butter over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant but not browned. Pour in the heavy cream and whole milk, then stir in salt, pepper, and dried thyme. Warm the mixture gently, whisking to combine all ingredients evenly. Remove from heat once warm.
Preheat your oven to 350°F (175°C). Butter your baking dish lightly to prevent sticking. Arrange a layer of potato slices on the bottom, slightly overlapping. Sprinkle a generous handful of shredded cheddar and mozzarella cheese over the potatoes. Repeat layering potatoes and cheeses until all are used, finishing with a layer of cheese on top. Pour the warm cream and garlic mixture evenly over the assembled potatoes, allowing it to seep through the layers.
Sprinkle the grated Parmesan cheese over the top layer. This will form a beautifully golden, crispy crust during baking.
Cover the dish tightly with aluminum foil and bake for 45 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork. Let the dish rest for 10 minutes before serving to let the sauce thicken slightly.
