Swiss chard is like two vegetables in one. Most recipes recommend removing the stems and saving them for another use, but very few recipes exist for chard stems alone, so they often go unused. I like to use the whole vegetable in my recipes. The stems need to be cooked longer, so I chop them and cook them along with the onions at the beginning and add the leaves near the end.
If chard is unavailable, kale makes a good substitute, though you may not need the balsamic vinegar, which I use to cut the bitter flavor of the chard.