How to Make It
Preheat the oven to 400°. Grease a 2-quart soufflé dish with 2 tablespoons of the butter and coat with the breadcrumbs, shaking out the excess.
In a medium saucepan, combine 1 1/2 cups of water with
3/4 teaspoon of salt and bring to a boil. Add the chopped broccoli, cover and simmer over moderate heat until very soft, 8 to 10 minutes. Stir in the spinach and cook until just wilted, about 30 seconds. Using a slotted spoon, transfer the broccoli and spinach to a food processor and puree until nearly smooth; cool.
Meanwhile, wipe out the saucepan and melt the remaining 5 tablespoons of butter in it. Add the shallots and cook over moderate heat, stirring, until softened, about 2 minutes.
Add the flour and cook, whisking, until the roux starts to change color, about 3 minutes. Gradually whisk in the milk and bring to a boil. Cook over moderately low heat, whisking constantly, until thick, about 3 minutes. Scrape the mixture into a large bowl and whisk in the cayenne, black pepper and 1 1/4 teaspoons of salt; let cool completely, about 15 minutes. Whisk in the cooled broccoli puree. Whisk in the egg yolks one at a time, then fold in the cheese.
In a large stainless steel bowl, using a hand mixer, beat the egg whites with the cream of tartar at medium-high speed until firm peaks form, about 2 minutes. Fold one-third of the whites into the soufflé base to lighten it, then fold in the remaining whites until no streaks remain.
Scrape the mixture into the prepared soufflé dish. Run your thumb around the dish’s inside rim to wipe away any crumbs. Bake for 45 to 50 minutes, until the soufflé is puffed and well browned. Serve right away.