Chop the chard, slicing the stems and roughly chopping the greens. Rinse well in a colander, then shake the colander to get rid of excess moisture.
Heat a large pot over medium high heat. Add the olive oil, then add the onions and sauté for 4 minutes. Add the garlic and the chard, season with salt and pepper and sauté for about 5 minutes, until the chard is crisp-tender.
Meanwhile, preheat the broiler. Beat the eggs, and season them with salt and pepper. Heat an 8 to 10 inch skillet over medium heat, melt the butter, then add the eggs, stirring frequently for two minutes. Stir in the cooked Swiss chard, then stir in half of the feta. Lower the heat to medium low and cook for 2 to 3 minutes until the eggs are set on the bottom. Sprinkle the rest of the feta over the top of the eggs, and transfer to the broiler. Broil for 1 to 2 minutes until the top is set and the frittata is lightly puffed.
Serve hot or at room temperature.
Notes
You could use other leftover sauteed greens in this as well, (and yes, of course, even kale) though the chard has nice texture, which even sautéed holds up well. The flavor is nice and light, very uncomplicated.