2 cups heavy cream
¼ tsp. nutmeg
1∕8 tsp. cayenne
2 garlic cloves, crushed
1 sprig thyme
4 sage leaves, chiffonaded, plus 6 whole sage leaves
Salt and freshly ground black pepper to taste
3 russet potatoes (about 2¼ lbs.)
2 parsnips (about 1 lbs.)
5 oz. freshly grated Parmesan cheese, divided
5 oz. freshly grated Gruyere cheese, divided
Grape seed oil or other neutral-flavored oil
Preheat the oven to 375 degrees.
In a saucepan, pour in the heavy cream, nutmeg, cayenne pepper, garlic, thyme, the chiffonaded sage leaves, and salt and pepper to taste. Bring the cream to a simmer. Immediately remove from heat. Discard the garlic.
Peel the potatoes and parsnips. Using a mandoline or a food processor fitted with the slicing blade, slice the potatoes into 1∕8-inch thick slices and set aside. Do the same with the parsnips.
In a 9-by-13-inch baking dish, spoon some of the cream in the bottom to prevent the gratin from sticking. Place the potato slices in a single layer in the bottom of the dish. Season the potatoes with a sprinkling of salt and pepper. Then spoon about 4 tablespoons of the cream evenly over the top. Sprinkle 1 ounce of each cheese on top.
Repeat this process with the parsnips and continue layering by alternating between the potatoes and parsnips with the remaining ingredients, but do not put cheese on the very top layer. You should have 3 layers of potatoes and 2 layers of parsnips.
Bake, uncovered, for 50 to 60 minutes, or until the potatoes and parsnips are hot and cooked through. (DO AHEAD:: Everything up to this point in the recipe can be made 1 day ahead, if needed. After removing the gratin from the oven, let it cool completely. Cover with foil and place in the refrigerator overnight. The next day, bring it back to room temperature, then let it bake in a 350-degree oven for 20 to 30 minutes, or until warmed through.)
While the gratin is baking, heat ¼ inch of grape seed oil in a pan over medium-high heat. Gently place the remaining 6 whole sage leaves in the oil for about 5 seconds. Immediately transfer to a paper towel-lined plate. Sprinkle with salt.
Remove the gratin from the oven and preheat the broiler. Top the gratin with the remaining 1 ounce of each cheese, and broil for 2 to 3 minutes, or until golden.
Remove the gratin from the oven and garnish with the fried sage leaves.