Heat oven to 400 degrees. Butter a 9-inch round or oval baking dish.

Slice potatoes 1/8-inch thick and combine thoroughly in a mixing bowl with 1/3 cup Gruyere, the olive oil, thyme, salt, and pepper.

Arrange potatoes in a neat overlapping pattern in the baking dish. Pour chicken stock over potatoes and sprinkle remaining cheese over top. Bake until potatoes are tender and the cheese and potatoes are starting to brown, about 45 minutes. Garnish with thyme sprigs.