Preheat oven to 375 degrees. Wash and dry Swiss chard. Cut off the very ends of the stems. Roughly chop (leaving stems intact) the chard.
In a saute or frying pan add onion and chard to the oil and saute until stems are tender (do not overcook). Salt and pepper to taste. Grate cheese to make 2 1/2 cups. Use whatever varieties you want/have. Be creative! I used Swiss, Cheddar, and Parmesan. Whisk eggs. Add milk and cheese. Fold in the onion/chard mixture. Season with salt and pepper to taste, if necessary. Pour into a Pyrex rectangular baking dish that has been sprayed with nonstick cooking spray. Bake for 35-45 minutes or until golden brown and no liquid seeps out when knife is inserted. Serve with crusty bread and a salad.