After cleaning chard, separate leaves from stems and coarsely chop leaves. Slice stems into ¼ inch pieces.
Over medium heat, place olive oil in a pot large enough to hold the chard. Add onions and cook for a minute or two. Add chard stems and mushrooms. Cook until softened, about 5 minutes.
Stir in chard leaves and season with salt and pepper. Cover pot, lower heat and braise for about 20 minutes, stirring occasionally.