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1
Combine all ingredients in a medium bowl and mix until the butter is incorporated.
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2
Cover and place in the refrigerator until ready to use.
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3
Heat the oven to 400 degrees F and arrange a rack in the middle.
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4
Coat a 13-by-9-inch baking dish with butter and set aside.
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5
Prepare the squash: Peel off the skin with a vegetable peeler.
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6
Trim the top and bottom.
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7
Cut the neck from the bulb of the squash.
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8
Halve each piece lengthwise and scrape out the seeds.
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9
Pour the cream into a large bowl and set aside.
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10
Slice the squash and celery root into 1/4-inch-thick pieces with a mandoline or sharp knife, placing the pieces in the cream as they are cut.
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11
Toss until well coated.
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12
Melt the butter in a medium frying pan over medium heat.
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13
When it foams, add the onion and season well with salt and pepper.
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14
Saute until soft and translucent, about 5 minutes; set aside.
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15
Construct the gratin by ladling a third of the squashcelery root mixture into the prepared baking dish, then seasoning well with salt and pepper.
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16
Top with half of the onions, then another layer of squash and celery root.
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17
Season the second layer with salt and pepper and cover with the remaining onions.
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18
Place the last of the squash and celery root on top and press down to create an even surface.
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19
Season with salt and pepper, then pour the remaining cream over top.
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20
Sprinkle the streusel evenly over the gratin.
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21
Bake until the vegetables are soft and the streusel is golden brown, about 40 to 50 minutes.
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22
Let sit at room temperature for about 20 minutes before serving.
