1 large butternut squash (about 1 pound)
about 1 tsp extra-virgin olive oil
3 Tbl butter, plus more for greasing the dish
3 Tbl all-purpose flour
1 1/2 cups whole milk, warmed
kosher salt & freshly ground black pepper, to taste
1 cup coarsely grated Gruyere cheese
1 Tbl light brown sugar
1/4 tsp cayenne pepper
3/4 tsp grated nutmeg
3 large egg yolks
4 large egg whites
kosher salt & freshly ground black pepper, to taste
First, roast and puree the butternut squash: Preheat the oven to 350°F. Cut the butternut squash in half lengthwise, scoop out the seeds, drizzle the insides with olive oil and sprinkle with a good pinch of salt and a few grinds of pepper. Place cut side-down on a parchment-lined baking sheet. Cover tightly with foil, and roast until completely tender, about an hour to an hour and a half. Remove from the oven, and when cool enough to handle, scoop out the tender flesh and discard the skins. Place the butternut flesh in the bowl of an electric food processor, and process until very smooth.
Using a kitchen scale, weigh out 12 ounces of the pureed squash. You’ll have a little left over, which, with a little added butter and brown sugar, will make for a tasty snack or side.
You can roast and puree the squash a day ahead, and keep refrigerated in an air-tight container. In fact, I’d definitely advise it!
Now, proceed with the soufflé: Pre-heat the oven to 425°F, and arrange a rack in the middle position. Generously butter a 2- or 3-quart shallow ceramic or glass baking dish.
Melt the butter in a medium-sized saucepot set over medium heat. Add the flour and whisk to combine. Cook, whisking constantly, for 2 minutes. Add the milk, a little at a time, and whisking as you pour. Bring to a boil, then lower the heat to maintain a simmer. Simmer, whisking occasionally, until thickened, about 5 minutes. Add the pureed squash, grated cheese, brown sugar, cayenne and nutmeg. Season with a big pinch (almost a teaspoon full) of kosher salt and a good pinch of pepper, then whisk to combine.
Place the egg yolks in a large mixing bowl and whisk. Add about a cup of the squash mixture and immediately whisk to combine. (If you don’t do this immediately, the hot squash mixture will start to cook the egg.) Whisk in about another cup of squash mixture, then add the remaining squash mixture to the yolks and whisk to combine. Set aside.
Combine the egg whites with a pinch of salt in the bowl of an electric stand mixer fitter with the whisk attachment. Beat on medium-high speed until the whites just hold stiff peaks. Fold a quarter of the whites into the squash mixture with a rubber spatula. Then fold in the remaining whites, working gently but thoroughly. Pour the mixture into the buttered baking dish. Bake, uncovered, for 15 minutes. Then loosely cover with foil (just drape it over the dish) and bake until puffed, golden brown and just set, about 30 minutes. Serve immediately.
