Hasselback Potato Gratin Recipe

Name: Hasselback Potato Gratin Recipe
Ingredients: 3 ounces (85g) finely grated Gruyère or Comté cheese; 2 ounces (60g) finely grated Parmigiano-Reggiano cheese; 2 cups (480ml) heavy cream; 1 tablespoon fresh thyme leaves, roughly chopped; 2 medium cloves garlic, minced; Kosher salt and freshly ground black pepper; 3 to 3 1/2 pounds (1.4 to 1.6kg) russet potatoes, peeled and sliced 1/8 inch thick on a mandoline slicer (5 to 6 medium potatoes; see note); 2 tablespoons (30g) unsalted butter