Cook's notes: 'The slight bitterness of mustard greens is cloaked in the smooth elegance of this superlative classic souffle.... Be sure to serve it immediately after baking, before it loses its airy puff.' 6 tablespoons (3/4 stick) unsalted
butter, at room temperature 2 tablespoons dried bread crumbs 6 oz. mustard greens (1 small bunch) 1 small onion, minced 1 garlic clove, minced 2 cups chicken broth, homemade or
canned 2 tablespoons cider vinegar 1/4 cup all-purpose flour 1 1/4 cups milk, heated 1 cup freshly grated Parmesan cheese 1/4 teaspoon cayenne pepper 1/8 teaspoon grated nutmeg 6 large eggs, separated, at room
temperature 1/8 teaspoon cream of tartar
Position a rack in the center of the oven and preheat to 400 degrees F. Lightly butter a 2 1/2-quart souffle dish with 1 tablespoon of the butter. Dust it with the bread crumbs; tap out excess crumbs.
In a large sink of lukewarm water, agitate the mustard greens well to remove any hidden grit. Drain in colander. Repeat. Remove and discard woody stems. Tear the greens into 2- to 3-inch pieces.
Heat one tablespoon of butter in medium saucepan. Add onion and garlic, and cook over medium heat, stirring often, until onion has softened. Add chicken broth and bring to boil. Add mustard greens and vinegar, reduce heat to medium-low, and simmer, covered, until greens are tender, about 30 minutes. Drain greens and chop finely, either by hand or in food processor. (The greens can be prepared a day ahead; allow to cool, then cover and refrigerate.)
In a heavy medium saucepan, heat remaining 4 tablespoons butter over low heat. Whisk in flour. Cook, stirring constantly for 2 minutes, without allowing mixture to brown. Whisk in hot milk and bring to boil. Reduce heat to low and simmer, whisking often, for about 5 minutes, until thickened. Stir in Parmesan cheese, salt, cayenne, nutmeg. Transfer souffle base to large bowl and allow to cool until lukewarm, stirring often, about 5 minutes. (The souffle base can be prepared a day ahead; allow to cool and then cover with plastic wrap pressed directly on the surface to prevent crusting, and refrigerate. Reheat in a saucepan over medium-low heat, stirring constantly, just until warm, before proceeding.)
Whisk egg yolks, one at a time, into souffle base. Stir in cooked greens.
Using a hand-held electric mixer set at low speed, beat the egg whites in a grease-free medium bowl until foamy. Add cream of tartar, increase speed to high, and beat just until soft peaks form. Stir about one-fourth of beaten whites into souffle base to lighten the mixture, then gently fold in remaining whites. Transfer mixture to prepared souffle dish.
Place dish in oven and immediately reduce heat to 375 degrees F. Bake until the souffle is puffed and golden brown, and a long broom straw inserted into the puff comes out clean, 35 to 40 minutes. Serve immediately. Serves 6 to 8.