-
1
Heat the oven to 400 degrees F and arrange a rack in the middle.
-
2
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.
-
3
Bring the water to a simmer in a large frying pan over medium heat.
-
4
Add the broccoli, season with salt and pepper, and stir to combine.
-
5
Cover with a tightfitting lid and cook, stirring occasionally, until the broccoli is crisp-tender, about 10 minutes.
-
6
Meanwhile, line a baking sheet with a double layer of paper towels.
-
7
When the broccoli is ready, drain it in a colander.
-
8
Transfer it to the prepared baking sheet, spread it into an even layer, and set aside.
-
9
Wipe out the frying pan with paper towels, add the measured butter, and melt over medium heat.
-
10
Add the onion and cook, stirring occasionally, until softened, about 10 minutes.
-
11
Add the flour, measured salt, and measured pepper and cook, stirring often, until the flour has darkened slightly in color, about 5 minutes.
-
12
While whisking constantly, slowly add the milk to the flour mixture until evenly combined and smooth.
-
13
(It will get very thick when you first add the milk, then thin out.)
-
14
Increase the heat to medium high and cook, whisking occasionally, until the mixture comes to a full simmer and thickens slightly, about 5 minutes.
-
15
Remove the pan from the heat and whisk in the remaining 2 cups of Parmesan.
-
16
Whisk in the egg yolks until smooth and combined.
-
17
Transfer the Parmesan sauce to a large bowl.
-
18
Transfer the reserved broccoli to a food processor fitted with a blade attachment and add 1/2 cup of the Parmesan sauce.
-
19
Process, stopping and scraping down the sides of the food processor as necessary, until smooth, about 1 minute.
-
20
Transfer the broccoli mixture to the bowl with the Parmesan sauce and stir to combine.
-
21
Taste and season with salt and pepper (the mixture should be slightly salty); set aside.
-
22
Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment.
-
23
Whisk on high speed until stiff peaks form, about 2 minutes.
-
24
Transfer about a third of the egg whites into the broccoli mixture and stir with a rubber spatula to incorporate.
-
25
Add the remaining egg whites and gently fold until thoroughly combined and no big streaks of egg white remain, being careful not to overmix.
-
26
Transfer the mixture to the prepared baking dish and spread it into an even layer.
-
27
Bake until puffed, golden brown, and just set, about 40 minutes.
-
28
Serve immediately.
