Chop and clean Swiss Chard: Stack several pieces of Swiss chard on work surface. Remove stems and set aside. Roughly cut leaves into pieces about 2-inch square. Repeat with the remaining swiss chard. Transfer the chopped leaves to a salad spinner filled with water. Drain, repeat washing if necessary, and spin dry. Rinse and chop the Swiss chard stems (about the same size as the diced onion.)

Heat oil in a large heavy skillet over medium high heat. Add chopped chard stems, garlic, onion, salt, thyme, nutmeg and pepper and cook, stirring often until the onions are starting to brown, 6 to 8 minutes. Add chopped cleaned Swiss chard leaves, 2 tablespoons water and cover. Let wilt, 2 to 4 minutes. Remove lid and continue cooking, stirring occasionally until the Swiss chard is completely wilted and softened, 1 to 3 minutes. Remove from the heat and drizzle with balsamic vinegar if using. Serve hot.