Wash the chard very well. Shake chard to dry it slightly, but leave a bit of the water on the leaves. Chop the stem and leaves into 2 inch pieces.
In a large skillet, heat the olive oil on medium heat. Add the chopped onions and saute for a couple minutes. Add the chard stalks; cook for a couple of minutes, then add the leaves and the garlic. If the leaves don't all fit in the pan, don't panic...just let them cook for a minute (as they cook it will "shrink" substantially), then add the rest. Continue to saute the chard until the leaves begin to wilt (a couple minutes. You will be surprised how much the chard reduces in size.
Swiss Chard With Onions And Garlic https://www.thegratefulgirlcooks.com/swiss-chard-onions-garlic/