Preparation and Cooking

• Start by arranging a rack in the top third of your oven and preheat it to 400°F. Grease a 9x13-inch broiler-safe baking dish with butter.
• In a large pot, combine the peeled and cut potatoes with 2 tablespoons of kosher salt. Cover with cold water and bring it to a boil. Cook for 7 to 10 minutes until just tender, then drain well.
• Melt 1 tablespoon of unsalted butter in a medium saucepan over medium heat. Add the minced garlic and sauté for about 2 minutes, until fragrant.
• Pour in the heavy cream and stir in the Dijon mustard, remaining kosher salt, black pepper, and nutmeg. Cook until it begins to simmer gently.
• Reduce heat to low, add 4 ounces of cheddar and 3 ounces of Parmesan cheese. Mix until melted and smooth, then remove from heat.
• Layer the drained potatoes evenly in the greased baking dish. Pour the cream mixture over the potatoes, ensuring they are well-coated.
• Sprinkle the remaining cheddar and Parmesan cheese over the top of the gratin.
• Place your dish in the oven and bake until the sauce is bubbling and the cheese turns golden brown, about 20 to 25 minutes.
• Switch your oven to broil on high and broil for an additional 2 to 4 minutes, then remove and let cool for 10 minutes.
• Optional: Garnish with chopped fresh chives or parsley.