The first time we made a potato gratin in culinary school I thought it sounded so fancy, but it was a cinch to make. The only thing that was missing for me was a super crunchy topping. My family in the Midwest uses cornflakes on top of potato casseroles, so the crispy cereal seemed like a fun, nostalgic topping for this elegant creamy potato dish.
Preheat the oven to 400 degrees F and coat the inside of a 9-inch cast-iron skillet with the butter.
Combine the cream, 1/4 cup Parmesan, thyme, bay leaves, garlic, nutmeg, 1 teaspoon salt and a few good grinds of pepper in a small saucepan over medium heat. Bring the mixture to a gentle simmer and cook, stirring occasionally, until the garlic and thyme are fragrant, about 2 minutes.
Discard the thyme and bay leaves with tongs. Spoon about 1/4 cup of the cream mixture into the bottom of the prepared skillet. Arrange about one quarter of the sliced potatoes on top of the cream in an even layer, then spoon another 1/4 cup of cream mixture over the top. Repeat until all of the potatoes and sauce are used. Sprinkle the remaining 1/4 cup Parmesan evenly over the top, then finish with an even layer of cornflakes.
Bake until the sauce is bubbling, the potatoes are tender when pierced with a knife and the cornflakes are golden brown, about 40 minutes. Sprinkle with more thyme for serving if desired.