-Preheat the oven to 350.

-Rub a 5- or 6-cup shallow gratin dish (or a 9.5 inch round pie plate) with some butter.

-Chop up 1½ tsp fresh thyme.

-Finely grate ¼ cup Parmigiano Reggiano.

-Make ¾ cup breadcrumbs by putting 2 pieces of stale bread in a food processor and chopping well.

-In a small bowl, combine the following:

1 TBS extra-virgin olive oil

The ¾ cup fresh breadcrumbs

½ tsp of the chopped fresh thyme

2 TBS of the finely grated Parmigiano Reggiano

-In a liquid measuring cup combine the following:

¾ cup heavy cream

½ cup low-sodium chicken broth

1 tsp Dijon mustard

-Cut 1 pound potatoes in half lengthwise and turn the halves cut side down on a cutting board.

-With a sharp knife, slice the halves across as thinly as you can (between 1/16 and ⅛ inch is ideal) so that you have thin half-moon pieces.

-Peel then cut 1 celery root into quarters and slice it as thinly as the potato.

-Put the potatoes and celery root in a mixing bowl and then add the following:

1/2 tsp sea salt

Several grinds of fresh pepper

3/4 cup grated Gruyere cheese

The remaining Parmigiano

The remaining 1 tsp thyme

The cream mixture

-Mix well.

-Using your hands, lift the potatoes out of the bowl and transfer them to the gratin dish, arranging them as evenly as possible.

-Pour and scrape the liquids and anything remaining in the bowl into the gratin dish and distribute everything evenly, adjusting the potatoes as necessary to get an even top.

-Using your palms, press down on the potatoes to bring the liquids up and around them as much as possible. (It won’t necessarily completely cover them.)

-Cover the top evenly with the breadcrumb mixture.

-Bake until the potatoes are tender when pierced with a fork (check the middle of the dish as well as the sides), the breadcrumbs are brown, and the juices around the edges of the gratin have bubbled down and formed a dark brown rim around the edge, 55 to 65 minutes.

-Let cool for about 15 minutes before serving.