Preheat the oven to 400°F and generously butter a medium baking dish or roasting pan. Arrange half of the celery root, potato and apple slices in the pan. For this first layer, you don’t need to arrange the slices very artfully as they won’t be seen, just be sure to distribute them evenly over the bottom so each bite contains some celery root, apple and potato. Generously salt and pepper this layer. Stir the crème fraîche to loosen the consistency then pour half over the top. Spread it to the edges with a flexible spatula, if necessary. Crumble half the blue cheese over the crème fraîche.
For the top layer, neatly arrange the slices of celery root, potato and apple, overlapping them as necessary to fit them in your pan. Generously salt and pepper this layer then cover with the remaining crème fraîche and blue cheese.
Bake the gratin for 20 minutes then cover with aluminum foil and bake until the potatoes are tender (they take the longest to cook through), about 30 minutes more. Check for doneness with a paring knife; you should feel almost no resistance when you insert the knife into the gratin. Take off the aluminum foil and continue to bake until the top is deep golden brown, about 15 minutes longer. Remove the gratin from the oven and let rest for 10-15 minutes. Sprinkle with chopped chives just before serving.
Notes: If you haven’t worked with celery root (also called celeriac) before, note that they have a gnarled root end that is almost always full of dirt and will have to be cut off completely. I call for a 1 ¼ to 1 ½ pound celery root to make up for lost root end. Once your celery root is cleaned, it should weigh somewhere between 14 oz and 1 lb. But a gratin is a very accommodating thing; a little more or less celery root, potato or apple won’t hurt anything.
- You can make this in a 13 x 9-inch baking dish if you like. The gratin will be thinner and there will be more of the crispy, golden-brown top to go around. This is an especially good option if you’re going to serve the gratin as part of a holiday buffet for a lot of people. I prefer to make it in a smaller roasting pan (the one pictured measures 8 x 12 inches), which yields thicker slices.
- This dish has always been a crowd-pleaser for me and I’ve had many sworn blue cheese haters ask for seconds. If you’re serving this to anyone who claims to dislike blue cheese (!), I have two recommendations: First, use a mild, creamy and salty blue cheese rather than a dryer, more pungent one. I recommend Fourme d’Ambert or Bleu d’Auvergne. These two French blues are delicious, fairly mild and have the added bonus of being inexpensive as far as cheeses go. A second, sneakier, option is to fail to mention that the gratin contains blue cheese. A lot of people never even realize it's there.
- If you really, REALLY hate blue cheese, just substitute another sharp, salty cheese. Sharp white cheddar or an aged gruyère would be delicious.