In a large skillet, melt 2 tablespoons of the butter. Add the onion and season with salt. Cover and cook over low heat, stirring a few times, until the onion is translucent, about 5 minutes. Stir in the chard, cover and cook, stirring a few times, until heated through, about 3 minutes. Add the tomato puree and stir until hot. Remove from the heat and stir in the remaining 2 tablespoons of butter. Season with salt and pepper and serve.