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