Tear Swiss chard leaves from thick white stalks and coarsely chop leaves, reserving stalks. In a large saucepan or kettle of boiling salted water simmer stalks tender, 5 to 10 minutes, and drain in a colander. Chard may be prepared up to this point 1 day ahead, stalks cooled completely and leaves and stalks cooled completely and leaves and stalks chilled separately in sealable plastic bags. Mince garlic. In a large skillet heat oil moderate heat until hot but not smoking and cook garlic, stirring, 30 seconds. Add leaves in 2 batches, tossing to coat with oil and stirring after each addition, and cook until leaves are wilted. Add stalks and cook, stirring occasionally, until heated through. Season chard with salt and pepper.