Preheat the oven to 400 degrees F.

Set a large skillet over medium heat and add 2 tablespoons of butter to the pan. Once the butter has melted, saute the onions in the pan, stirring often until the onions are caramelized, about 30 minutes. Season the onions with 1/2 teaspoon of salt and 1/4 teaspoon of the pepper. Set the onions aside to cool. Using the same skillet, melt another 2 tablespoons of butter over medium high heat, and saute about 1/4 of the potatoes until they are lightly caramelized, about 5 minutes. Season the potatoes with 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper. Set the potatoes aside to cool while you continue to cook the remaining potatoes. Repeat this process with the remaining butter, potatoes, salt, and pepper. Once all of the potatoes have been cooked and cooled, place 1/4 of them in a slightly overlapping single layer into a 9 by 13-inch casserole pan. Sprinkle a third of the caramelized onions over the potatoes, and sprinkle with 1 teaspoon of the chopped thyme. Place another layer of the potatoes on top of the onions, another third of the onions, and another teaspoon of the thyme. Repeat this process with the remaining potatoes, onions, and thyme, finishing with a top layer of potatoes.

Pour the chicken stock over the potatoes and place the casserole in the oven. Bake the casserole for 25 minutes, reduce the oven temperature to 350 degrees F, and continue to cook the casserole until the potatoes are golden brown, about 30 minutes more.

Remove the casserole from the oven, and allow to cool for 10 minutes before serving.