Preheat the oven to 350 degrees.

Heat the cream, garlic, and thyme sprig in a medium saucepan until bubbles begin to form around the edge of pan. Remove the pan from heat; set aside to steep.

Melt 1 tablespoon of the butter in a skillet over medium heat. Add the leeks, season with salt and cook, stirring often, until they are tender (do not brown), 10-12 minutes. Transfer to a small bowl and set aside.

Butter a 3 quart gratin dish (or any casserole) with the remaining 1 tablespoon of butter. Layer 1/3 of potato slices and 1/3 of the celery root slices evenly over the bottom of the baking dish. Cover with 1/3 of the leeks, then 1/3 of the Gruyere cheese. Sprinkle with salt, pepper, and 1 teaspoon of thyme leaves. Repeat layers twice more.

Strain the cream mixture into a pitcher and pour it over the vegetables.

Set the gratin dish on a large rimmed baking sheet and cover tightly with foil. Bake for 1 hour.

Carefully remove the foil and continue baking until the top is golden brown and the sauce is bubbling, 25-30 minutes.

NOTE: This can be made two hours ahead. Let it stand at room temperature. To serve, tent it with foil and reheat in a 300 degree oven until it is hot, about 20 minutes.

Submitted by Carnegie Hill CSA Judi Katz-Schwartz,Yoo Mi Thompson