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For the streusel:
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1. Combine all ingredients in a medium bowl and mix until butter is incorporated. Cover and place in
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the refrigerator until ready to use.
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For the gratin:
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1. Heat the oven to 400u00b0F and arrange a rack in the middle. Coat a 13-by-9-inch baking dish with
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butter and set aside.
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2. Pour cream into a large bowl and set aside. Slice squash and celery root into 1/4-inch-thick pieces
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with a mandoline or sharp knife, placing pieces into the cream as they are cut. Toss until well
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coated.
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3. Melt butter in a medium frying pan over medium heat. When it foams, add onion and season well with
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salt and freshly ground black pepper. Saute until soft and translucent, about 5 minutes; set aside.
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4. Construct gratin by ladling 1/3 of the squash and celery root mixture into the baking dish, then
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seasoning well with salt and freshly ground black pepper. Top with 1/2 of the onions, then another
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layer of squash and celery root. Season the second layer with salt and pepper and cover with the
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remaining onions. Place the last of the squash and celery root on top and press down to create an
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even surface. Season with salt and pepper, then pour the remaining cream over top.
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5. Sprinkle streusel evenly over gratin. Bake until vegetables are soft and streusel is golden brown,
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about 40 to 50 minutes.
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6. Let sit at room temperature for about 20 minutes before serving.
