Heat oven to 350.
Rinse chard and drain in a colander; this is so some water will cling to the leaves. In microwave-safe container with a lid, micro cook chard for 5 minutes on high. Do this in two batches if your container wont hold it all at once. Set cooked chard aside.
In large mixing bowl, whisk eggs, flour, milk, salt and baking powder until well combined. Add garlic, roasted red peppers, pesto and onion and stir to combine. Stir in cheese and chard (drain first if any water has accumulated), mixing well.
Put butter in a 13- by 9-inch baking dish and place in oven. In 2 or so minutes, when butter has melted, remove dish from oven, swirl to coat bottom of dish with butter, pour chard mixture into hot dish and return to oven. Bake 40-45 minutes.
Remove from oven, let cool 5 minutes, cut into pieces and serve.
Note: If you want to use this as an appetizer, you may cut it into 48-60 small squares. This can be made a day ahead; in fact, it tastes great as leftovers.