3 ounces finely grated Gruyère or comté cheese
2 ounces finely grated Parmigiano-reggiano
2 cups heavy cream
2 medium cloves garlic, minced
1 tablespoon fresh thyme leaves, roughly chopped
Kosher salt and freshly ground black pepper
3 to 3 1/2 pounds russet potatoes, peeled and sliced 1/8th-inch thick on a mandoline slicer (5 to 6 medium, see note above)
2 tablespoons unsalted butter