-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
-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.