Bake the pie crust according to package directions.
Wash Swiss chard and shake dry. Chop stems into 1/4 inch pieces and slice leaves into ribbons.
Heat Olive oil in a skillet over medium high and add onions and chard stems. Season. Cook until onion is soft and beginning to brown, about 10 minutes, stirring often.
Add lemon zest and chard leaves and cook about 4 minutes, until leaves have wilted, much like spinach.
Transfer cooked mixture to a colander and drain.
Place mixture into a large mixing bowl and stir in Cheddar, cream, egg, nutmeg.
Turn into baked shell and bake at 350 for about 45 minutes or until filling is set.
Top with Parmesan immediately when hot out of oven. Let tart sit for 10 minutes before slicing. Serves 4 for main dish, 6-8 for a side dish.