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 stir for another minute. Add the chard, season with salt and pepper and sauté for about 5 minutes, until the chard is crisp-tender (if you like it softer, cook it a bit longer). Serve hot.