Parmesan and Broccoli Casserole

Ingredients

• 0.3 teaspoon pepper black as needed freshly ground plus more
• 20 ounces broccoli cut into 1-inch florets
• 0.3 teaspoon cream of tartar 
• 6 large egg whites at room temperature
• 6 large egg yolk at room temperature
• 0.5 cup flour all-purpose
• 1.5 teaspoons kosher salt as needed plus more
• 2.5 cups parmesan cheese finely grated grated
• 8 tablespoons butter unsalted as needed plus more (1 stick)
• 0.5 cup water 
• 5 cups milk whole
• 1 medium onion yellow

Equipment

• food processor
• bowl
• frying pan
• baking sheet
• paper towels
• oven
• whisk
• baking pan
• stand mixer
• spatula
• colander

Directions

1. Heat 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.Bring the water to a simmer in a large frying pan over medium heat.
2. 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.
3. 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.
4. Add the onion and cook, stirring occasionally, until softened, about 10 minutes.
5. 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.
6. Remove the pan from the heat and whisk in the remaining 2 cups of Parmesan.
7. Whisk in the egg yolks until smooth and combined.
8. Transfer the Parmesan sauce to a large bowl.
9. 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.
10. 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.
11. Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment.
12. Whisk on high speed until stiff peaks form, about 2 minutes.
13. Transfer about a third of the egg whites into the broccoli mixture and stir with a rubber spatula to incorporate.
14. Add the remaining egg whites and gently fold until thoroughly combined and no big streaks of egg white remain, being careful not to overmix.
15. Transfer the mixture to the prepared baking dish and spread it into an even layer.
16. Bake until puffed, golden brown, and just set, about 40 minutes.
17. Serve immediately.