Wash the chard and place, with water still clinging to leaves, it in a large pot over high heat. Cook, covered, with just the water clinging to leaves, stirring occasionally, until wilted, about 6 minutes.  Press or squeeze out the excess liquid. 

Meanwhile, boil pasta until al dente and set aside.

Heat milk in a small saucepan over moderate heat, stirring, until warm. Keep warm. Meanwhile, cook onion in butter in a large pot over moderately low heat, stirring occasionally, until softened, about six minutes. Whisk in flour and cook roux, whisking, about three minutes. Add warm milk in a slow stream, whisking constantly to prevent lumps, and simmer, whisking, until thickened, three to four minutes.  Stir in the parmesan cheese while cooking sauce.  Stir in chard, then salt and pepper to taste and cook, stirring, until heated through.  Toss with pasta, add extra parmesan, if desired, and serve.