How to Make It
In a small bowl, cover the porcini mushrooms with hot water and let stand until they are softened, about 20 minutes. Lift the porcini from the water, squeeze dry and coarsely chop.
Meanwhile, preheat the oven to 350°. In a medium saucepan, melt 3 tablespoons of the butter over moderate heat. Stir in the flour until blended. Gradually whisk in the milk until smooth. Bring the béchamel to a boil over moderately high heat, whisking constantly until slightly thickened. Reduce the heat to low and simmer, whisking often, until no floury taste remains and the béchamel is very thick, about 20 minutes. Season with salt, pepper and nutmeg. Transfer the béchamel to a large bowl and, while it is still hot, whisk in the pecorino and egg yolks.
In a large skillet, cook the Swiss chard over moderately high heat in batches, one handful at a time, until the leaves are wilted, about 2 minutes per batch. Transfer the chard to a colander to cool, then squeeze dry. Coarsely chop the chard.
In the same large skillet, melt the remaining 3 tablespoons of butter. Add the Swiss chard and cook over moderate heat, stirring, until it's heated through and coated with butter, about 4 minutes. Season with salt and pepper. Stir the chopped chard into the béchamel along with the porcini mushrooms, 1 teaspoon of salt and 1/4 teaspoon of pepper.
Butter an 8-cup ceramic soufflé dish. In a large stainless steel bowl, beat the egg whites with a pinch of salt until firm peaks form. Fold one-third of the beaten whites into the chard mixture, then fold in the remaining whites. Scrape the batter into the prepared dish and run your thumb around the inside of the rim to help the soufflé rise evenly. Bake until the soufflé rises and is golden brown on top and slightly soft in the center, about 1 hour and 5 minutes. Serve right away.