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1
Preheat oven to 375 degrees F. Butter a 2-quart gratin dish or other ovenproof casserole.
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2
Cut off the entire rough exterior of the celery roots, making sure that all brown parts are removed.
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3
You will be removing about 1/4 to 1/2-inch of the exterior of each.
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4
Next, cut each celery root in 1/2 lengthwise.
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5
Turn one 1/2 cut side down, and cut into 1/4-inch wide pieces.
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6
Repeat with the remaining celery root.
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7
Set aside.
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8
Cut off the fronds and the dark green parts of each fennel bulb, leaving just the bulb.
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9
Cut each bulb in half lengthwise.
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10
Cut each half lengthwise into 1/4-inch thick slices.
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11
In a vegetable steamer, combine the celery root and fennel and steam for 5 minutes.
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12
Remove from the steamer and set aside to cool slightly.
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13
Cut the apples into 1/4-inch-thick rings.
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14
Place 1/3 of the celery root on the bottom of the prepared dish.
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15
Sprinkle with salt and pepper to taste.
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16
Overlap with 1/3 of the apple slices, and sprinkle with a little more salt but no pepper.
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17
Put 1/3 of the fennel over the apples, and once again sprinkle with salt and pepper.
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18
Repeat with the remaining celery root, apple, and fennel.
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19
Pour cream around the sides of the dish until the cream reaches about halfway up the dish.
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20
Sprinkle the top of the gratin with the cheese.
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21
Cover and bake for 35 minutes, or until the vegetables and apple feel tender when pierced with a fork or a skewer.
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22
Turn the oven to broil, remove the cover, and broil until the gratin is golden brown and bubbling, about 3 minutes.
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23
Let cool 15 minutes to allow the cream to firm up slightly and serve.
