Step 1Heat the oven to 400 degrees. Butter a 2 1/2-quart capacity, straight-sided baking dish and coat lightly but evenly with 2 tablespoons of the cheese or bread crumbs.
Step 2Melt the butter in a saucepan over moderate heat. Add the flour and cook, stirring, until the roux smells nutty but before it browns, about 2 minutes. Whisk in the milk and continue whisking until the sauce is as thick as paste. Remove the pan from the heat and whisk in the yolks one at a time. Season the souffle base with salt, pepper and the nutmeg. Generously season the spinach or chard and stir it into the souffle base with the remaining cheese or bread crumbs.
Step 3Add a dash of salt to the egg whites and beat them in a standing mixer with the whisk attachment until they are glossy and form supple peaks, about 4 minutes. Stir half of the egg whites into the souffle base mixture, then gently fold in the remaining egg whites. Pour the batter into the prepared dish, smoothing the top.
Step 4Place the dish on the middle rack and immediately turn the temperature down to 375 degrees. Bake the souffle without disturbing until it is puffed and the top is golden brown, 30 to 35 minutes.