Heat oven to 400° F.
In a large skillet, over medium-low heat, melt 2 tablespoons of the butter. Add the onion and cook for 5 minutes. Add the garlic and chard (if using) and increase heat to medium-high. Cook, stirring frequently, for 5 to 7 minutes. Season with 1 teaspoon of the salt, 1/2 teaspoon of the pepper, the sage, and the nutmeg.
In a large bowl, combine the pumpkin, cream, 1 cup of the Parmesan, and the remaining salt and pepper.
Place a layer of ravioli in a buttered 9-by-13-inch baking dish. Top with half the onion mixture and almost half the pumpkin mixture. Top with another layer of ravioli. Repeat with the remaining onion mixture and all but 1 cup of the pumpkin mixture. Top with the remaining ravioli, pumpkin mixture, and Parmesan. Sprinkle with the remaining butter, cut into small pieces. Cover and bake for 20 minutes. Uncover and bake for 15 minutes more.