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.
Notes
Store chard wrapped in slightly damp paper towels, then tucked into an open plastic bag, where it will last for up to 3 days.
I like to slice the stems and sauté them right along with the leaves. The stems will stay a bit crisper, but the combo of textures is very appealing.