Bring 4 ½ cups water to boil in a medium saucepan over medium-high heat. Pour the cornmeal slowly into the liquid, stirring with a wire whisk to prevent clumping.Continue stirring as the mixture thickens, 2 to 3 minutes.
Turn heat to low and continue cooking for 40 - 45 minutes, stirring every 5 - 10 minutes to prevent it from clumping.
When the polenta has 10 minutes remaining to cook, rinse and chop the Swiss chard into 1-2 inch squares. Heat olive oil in a large skillet. Add the garlic and sauté for 1 minute then add the Swiss chard. Cook, stirring, until the chard begins to wilt. Turn the heat to low, then cover and cook for 15 minutes.
When the polenta is cooked through and most of the water is evaporated, whisk in butter and Parmesan. Season to taste with salt and pepper.
After the chard has cooked for 15 minutes, uncover and cook a few minutes longer, stirring often. Stir in the green onions and season with salt and pepper to taste. Sprinkle with Parmesan. Serve over creamy polenta.