This impressive casserole recipe made with broccoli and cheese baked into a fluffy soufflélike texture is worth the effort. Make a creamy base of flour, milk, egg yolks, lots of Parmesan cheese, and puréed broccoli. Beat egg whites separately, then fold them into the base to give the casserole lift and lightness as it bakes. Serve this special side dish next to a prime rib or ham for the holidays.
- 1Heat the oven to 400°F and arrange a rack in the middle. Generously coat a 13-by-9-inch baking dish with butter, then evenly sprinkle the bottom and sides with 1/2 cup of the Parmesan; set aside.
- 2Bring the water to a simmer in a large frying pan over medium heat. Add the broccoli, season with salt and pepper, and stir to combine. Cover with a tightfitting lid and cook, stirring occasionally, until the broccoli is crisp-tender, about 10 minutes. Meanwhile, line a baking sheet with a double layer of paper towels.
- 3When the broccoli is ready, drain it in a colander. Transfer it to the prepared baking sheet, spread it into an even layer, and set aside.
- 4Wipe out the frying pan with paper towels, add the measured butter, and melt over medium heat. Add the onion and cook, stirring occasionally, until softened, about 10 minutes. Add the flour, measured salt, and measured pepper and cook, stirring often, until the flour has darkened slightly in color, about 5 minutes.
- 5While whisking constantly, slowly add the milk to the flour mixture until evenly combined and smooth. (It will get very thick when you first add the milk, then thin out.) Increase the heat to medium high and cook, whisking occasionally, until the mixture comes to a full simmer and thickens slightly, about 5 minutes.
- 6Remove the pan from the heat and whisk in the remaining 2 cups of Parmesan. Whisk in the egg yolks until smooth and combined. Transfer the Parmesan sauce to a large bowl.
- 7Transfer the reserved broccoli to a food processor fitted with a blade attachment and add 1/2 cup of the Parmesan sauce. Process, stopping and scraping down the sides of the food processor as necessary, until smooth, about 1 minute. Transfer the broccoli mixture to the bowl with the Parmesan sauce and stir to combine. Taste and season with salt and pepper (the mixture should be slightly salty); set aside.
- 8Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment. Whisk on high speed until stiff peaks form, about 2 minutes.
- 9Transfer about a third of the egg whites into the broccoli mixture and stir with a rubber spatula to incorporate. Add the remaining egg whites and gently fold until thoroughly combined and no big streaks of egg white remain, being careful not to overmix. Transfer the mixture to the prepared baking dish and spread it into an even layer.
- 10Bake until puffed, golden brown, and just set, about 40 minutes. Serve immediately.