In a large (12-inch) skillet, heat the olive oil over medium heat until shimmering. Add the onion and cook until softened, about 3 minutes, then add the garlic and cook for a minute more. Add the chard and a splash of water, cover, and cook until the chard is completely soft. Add more water if necessary if the skillet is too dry and the chard is still tough. Before proceeding, cook off any leftover moisture, and allow to cool.