Prepare the Swiss chard mixture: Heat oil in large skillet over medium heat. Add onions, cook and stir until translucent, about 5 minutes. Increase heat to medium high and add chopped Swiss chard, all of it, and press down and stir until wilted. Add salt, pepper, sage and nutmeg and continue cooking and stirring until no liquid is left in pan, 5 to 10 minutes. Preheat oven to 400º.
Meanwhile, while Swiss chard is cooking, brown the Italian sausage in another skillet over medium high heat. Break up the sausage while cooking into crumbles, stirring frequently until cooked through, 5 to 10 minutes. Drain grease and set sausage aside.
Make pumpkin mixtures: In medium bowl, combine 2 cups pumpkin, ¾ cup heavy cream, ½ cup Parmesan cheese, salt, pepper, sage and nutmeg (pumpkin/cheese mixture). Set aside. In a small bowl, combine remaining 1 cup of pumpkin with remaining ¾ cup heavy cream (pumpkin/cream mixture). Set aside.
Assemble lasagna: Pour milk into bottom of 9" x 13" baking dish. Place 3 noodles widthwise in the dish and spread ½ the pumpkin/cheese mixture over the noodles. Scatter ½ the sausage crumbles over the pumpkin, followed with ½ the Swiss chard mixture. Add second set of 3 noodles, top with remaining pumpkin/cheese mixture, sausage crumbles and Swiss chard mixture. Place last 3 noodles on top. Pour reserved pumpkin/cream mixture evenly over the top and letting some run down the edges. Sprinkle with 1 cup grated Parmesan and dot with the butter, cut into 3 pieces.
Cover carefully with aluminum foil and bake for 20 minutes. Remove foil and continue baking another 15 minutes more, until lightly golden. Let rest 10 minutes before cutting and serving.
I put under the broiler for a couple of minutes at the end of baking time for a more golden brown top. That is, of course, optional.