Melt the butter in a pan over medium heat. Add the onion and garlic and sauté for 5 minutes until aromatic.
Add the Swiss chard. Stir occasionally until it has wilted and softened, about 5–8 minutes.
Set the heat to medium-high. Add the cream and Parmesan. Stir until well mixed. Simmer for 5 minutes until the cream thickens. Season with salt and pepper.