Preheat the oven to 350°F. Butter a 3-quart glass casserole. Slice the leeks thinly lengthwise. Using a mandoline or sharp knife, cut the potatoes crosswise into round slices about ¹/₈ inch thick.

Place 1 layer of potato slices on the bottom of the dish and sprinkle with salt and pepper. Melt the 4 tablespoons butter in a large skillet over medium heat; add the thyme sprigs and leeks and sauté until the leeks are tender and golden, 5 to 7 minutes. Discard the thyme and scatter half the leeks and half the Gruyère over the potatoes. Drizzle with half of the cream.

Add the remaining cream and the bay leaf to the skillet with the remaining leeks and simmer for 5 minutes. Stir in the nutmeg. Layer the remaining potatoes in the casserole. Discard the bay leaf and spoon the leek-cream mixture evenly over the second layer of potatoes. Top with the remaining Gruyère and cover the casserole dish with aluminum foil.

Bake 40 minutes. Remove the foil and bake until the cheese is bubbling and golden, 15 to 20 minutes longer. Let cool for a few minutes before serving.