Ingredients

• 3 tablespoons salted butter
• 1 yellow onion, thinly sliced
• Kosher salt and black pepper
• 1/4 cup chicken stock
• 1 teaspoon fresh lemon juice
• 1 clove garlic, chopped
• 1 tablespoon chopped fresh thyme
• 2 cups heavy cream
• 1 cup whole milk
• 1 pinch cayenne
• 3/4 cup shredded Gruyère cheese
• 3/4 cup shredded cheddar cheese
• 6 russet or Yukon Gold potatoes, sliced 1/8 inch thick

Instructions

1. Preheat the oven to 350°F (175°C).
2. In a large skillet over medium-high heat, melt the butter in a large skillet over medium-high heat. Add the sliced onions and cook for about 10 minutes, stirring occasionally, until softened. Pour in the chicken stock and fresh lemon juice, then continue cooking for another 5–7 minutes, stirring occasionally, until the liquid has mostly evaporated and the onions become deeply golden and caramelized.
3. Stir in the chopped garlic and fresh thyme. Season with kosher salt and black pepper, then cook for an additional 2 minutes. Remove the skillet from the heat.
4. Butter a 9×13-inch casserole dish. In a large measuring cup, whisk together the heavy cream, whole milk, cayenne, salt, and pepper. Stir in the Gruyère cheese.
   Pour one-third of the cream mixture into the prepared baking dish. Arrange one-third of the potato slices over the liquid, followed by half of the caramelized onions. Add another third of the cream mixture, another layer of potatoes, and the remaining onions. Finish with the final layer of potatoes and pour the remaining cream mixture evenly over the top, making sure the potatoes are arranged in an even layer.
5. Cover the dish tightly with foil and bake for 1 hour 45 minutes to 2 hours. Remove the foil, sprinkle the cheddar cheese evenly over the top, and return the dish to the oven for 20 minutes, or until the cheese is melted and lightly browned. Garnish with fresh thyme and serve warm.

Make-Ahead Tip