Preheat the oven to 375º, and line a sheet tray with parchment paper.

Heat together the cream, roasted garlic and thyme in a saucepan until steaming. Set aside while preparing the vegetables.

Lightly butter a baking or pie dish. Place a layer of sliced potatoes on the bottom of the dish in a concentric circle. Season with salt and pepper to taste, ladle about 1/4 cup of the infused garlic cream on top, and top with a sprinkling of Gruyere cheese.

Arrange the next layer using celery root, and continue in this fashion, alternating potatoes and celery root, until the baking dish is full.

Top the gratin with Gruyere cheese and cover with foil. Bake on the prepared sheet tray for 40 minutes.

Uncover the baking dish, and continue to cook until the potatoes and celery root are knife-tender and the cheese is golden brown and bubbly.

Allow to cool for 10 minutes, cut into slices and serve.

NOTES:

These can be made individually in ramekins, which will take half the time to bake.

To roast garlic, slice the top off a whole head of garlic to expose the cloves. Place it in a small oven-proof dish, drizzle with a tablespoon of olive oil and cover with foil. Bake at 350º for 40 to 50 minutes, until the cloves are soft and a light golden color.