Preheat oven to 350°F. Lightly grease a 9- x 13-inch casserole dish.
Melt 3 tablespoons butter and evoo in a large sauté pan over medium heat. Sauté shallots and garlic for 3 to 5 minutes or until slightly limp and very fragrant.
Add squash purée and cook stirring constantly until squash is dry. Add cream and continue cooking for 5 to 10 minutes, until squash is smooth, creamy, and fairly dry.
Remove from heat and transfer to a large mixing bowl. Add parsley, cheese, nutmeg, salt, and pepper. Set aside to cool.
Fill a piping bag, fitted with a medium tip, with cooled squash mixture. Pipe into shells, filling them generously but not over stuffing. Place filled shells into prepared dish.
Melt 4 tablespoons of butter in a small saucepan over medium heat. Cook for 5 to 7 minutes, until butter is nut brown in color and very fragrant. Remove pan from heat and add sage leaves to hot butter.
Pour butter and sage over shells and bake at 350°F for 30 minutes. Garnish with Parmesan cheese before serving.