In a large nonstick frying pan, heat the oil over moderately low heat. Add the onions and cook, stirring occasionally, until translucent, about 5 minutes. Increase the heat to moderately high and add the chard, 1 teaspoon salt, 1/2 teaspoon pepper, 1/2 teaspoon sage, and 1/4 teaspoon nutmeg. Cook, stirring, until the chard is wilted and no liquid remains in the pan, 5 to 10 minutes.
Heat the oven to 400°. In a medium bowl, mix together 2 cups of the pumpkin, 3/4 cup cream, 1/2 cup Parmesan, and the remaining 1 1/4 teaspoons salt, 1/2 teaspoon pepper, 1/2 teaspoon sage, and 1/4 teaspoon nutmeg.
Pour the milk into an 8-by-12-inch baking dish. Top the milk with one third of the noodles, then spread half the pumpkin mixture over the noodles. Layer half the Swiss chard over the pumpkin and top with a second layer of noodles. Repeat with another layer of pumpkin, Swiss chard, and noodles. Combine the remaining 1 cup of pumpkin and 3/4 cup of cream. Spread the mixture evenly over the top of the lasagne, sprinkle with the remaining 1 cup of Parmesan, and dot with the butter. Cover with aluminum foil and bake for 20 minutes. Uncover and bake until golden, about 15 minutes more.