Ingredients

• 2 tablespoons unsalted butter
• 1 medium yellow onion, thinly sliced
• 4 sprigs fresh thyme, plus more for garnish
• 1 1/2 teaspoons kosher salt
• 1 teaspoon freshly ground black pepper
• 2 teaspoons all-purpose flour
• 1/4 cup dry white wine, such as Pinot Grigio
• 1 cup low-sodium beef broth
• 2 1/2 pounds large Yukon Gold potatoes (4 to 6), peeled and sliced 1/8-inch thick
• 4 ounces shredded Gruyère cheese (about 1 cup), divided
• 4 ounces finely grated Parmesan cheese (1 cup store-bought grated), divided
• 1/2 cup heavy cream

1. Heat the oven to 350°F.
2. Melt 2 tablespoons unsalted butter in a 10-inch cast iron or ovenproof high-sided skillet over medium heat. Add 1 thinly sliced medium yellow onion, 4 fresh thyme sprigs, 1 1/2 teaspoons kosher salt, and 1 teaspoon black pepper. Stir to combine, cover, and cook undisturbed for 5 minutes.
3. Uncover, stir, cover again, and cook for 5 minutes more. Uncover and continue to cook, stirring occasionally at first and then more frequently once the onions start to brown, until the onions are a rich caramel brown color, 8 to 15 minutes more. Reduce the heat as needed if the onions are browning too quickly.
4. Sprinkle 2 teaspoons all-purpose flour over the onions, stir well, and cook for about 1 minute to lightly toast the flour. Stir in 1/4 cup dry white wine and cook, scraping the bottom of the skillet, until absorbed, about 1 minute.
5. Remove the skillet from the heat. Remove and discard the thyme stems. Stir 1 cup low-sodium beef broth into the onions.
6. Arrange 2 1/2 pounds peeled and sliced Yukon Gold potatoes vertically in the skillet: Stack about 10 slices or 1 to 2 inches of slices at a time, then lay the stack on its side in the skillet and push down on it a little for a slight shingling effect.
7. Stir 2 ounces of the shredded Gruyère cheese and 2 ounces of the grated Parmesan cheese together in a small bowl. While using one hand to riffle through the potato slices (like flipping through pages in a filing cabinet), sprinkle the cheese mixture with the other hand so a little gets in between each slice. (Don’t worry about getting it in every crevice.)
8. Place the skillet on a rimmed baking sheet. Transfer to the oven and bake for 30 minutes. Meanwhile, place the remaining 2 ounces Gruyère and 2 ounces Parmesan in the now-empty cheese bowl. Stir in 1/2 cup heavy cream.
9. Spoon the cream and cheese mixture evenly over the potatoes. Return to the oven and bake until the top is deep golden-brown with dark brown splotches, and the potatoes are tender, 35 to 40 minutes more.
10. Let cool for 20 minutes so the sauce can thicken. Do not skip this step, as the sauce needs time to thicken up. Garnish with fresh thyme leaves before serving.