Easy Broccoli Casserole Recipe - Parmesan cheese

Method

• Heat the oven to 400 degrees 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.
• Bring 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.
• When 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.
• Wipe 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.
• While 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.
• Remove 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.
• Transfer 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.
• Place 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.
• Transfer 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.
• Bake until puffed, golden brown, and just set, about 40 minutes.
• Serve immediately.