I have this recipe in my files from so long ago that I no longer recall whether it was inspired by another recipe or I concocted it myself. What I do know is that I have made it my own. The sweet squash and earthy celery root are great complements, thyme keeps the squash on the savory side, and the gratin is colorful and full of flavor. For a leaner dish, substitute half-and-half, whole milk, or even 2% milk for the cream. - JSCooks —JSCooks
Test Kitchen Notes
The combination of butternut squash and celery root is a wonderful match -- not to mention a beautiful color contrast. A touch of thyme and onion complete this beautiful gratin. - Stephanie —The Editors
Butter or pan spray, for pan
Salt and freshly ground black pepper
butternut squash, approx. 1-1/2 pounds, peeled, cut in half lengthwise, and seeded
celery root, 3/4 to 1 pound, peeled and cut into quarters
Leaves pulled from several branches of thyme
large red onion, peeled and cut into thin slices
slices firm white bread, slightly stale, processed into coarse crumbs
finely chopped walnuts
grated pecorino, asiago, or parmigiano-reggiano cheese
Handful of flat-leaf parsley leaves, coarsely chopped
Preheat the oven to 450ºF. Butter or spray a 1-1/2 quart gratin dish.
Cut the flesh from the neck of the butternut squash into 1/8-inch-thick half-moons. Cut the seeded "bowls" into 1/8-inch-thick slices and set aside separately from the half-moons.
Cut the celery root quarters into 1/8-inch-thick slices.
Layer the vegetables in the prepared pan, sprinkling each layer lightly with salt and pepper, as follows: Make a single layer of squash half-moons in the bottom of the pan. Add a layer of celeriac, using about half of it. Scatter about 2/3 of the onion slices and about half of the thyme over the celeriac. Make another layer using the odd-shaped squash pieces and the remaining celeriac. Make a final, neater layer of overlapping squash half moons, then scatter the remaining onions and thyme over the top. Dot the butter over the top and pour the cream evenly over everything.
Cover the dish with foil and bake until the vegetables are quite tender but not mushy when pierced with a sharp knife, about 45 minutes.
Remove the foil and sprinkle the walnuts over the top, then the bread crumbs, and finally the cheese. Return the dish to the oven, uncovered, until the top is golden and most of liquid has evaporated, about 15 minutes longer.
To further brown the top, if desired, place the dish under the broiler for a few moments, taking care not to burn the gratin. Remove from the oven and let cool slightly, about 15 minutes. Sprinkle the parsley over the top of the gratin just before serving, or sprinkle parsley over individual servings.