Preheat the oven to 425°F. Lightly coat six 10 to 12-ounce individual casseroles with nonstick cooking spray.
In a 6-quart Dutch oven cook squash in lightly salted boiling water, covered, for 15 to 17 minutes or until tender when pierced with a fork. Drain; return to Dutch oven. Mash with butter, Parmesan, 1/2 teaspoon salt, and 1/4 teaspoon pepper; set aside.
In a large skillet cook sausage over medium-high heat, breaking up with the back of a wooden spoon until no longer pink. Transfer meat to a colander; drain well. Wipe out skillet with a paper towel.
In the skillet, heat oil over medium-high heat. Add onion and mushrooms; cook, stirring occasionally, until tender, about 4 to 5 minutes. Add garlic; cook 2 minutes more. Stir in broth, crushed tomatoes, rosemary, and Worcestershire sauce. Bring to boiling; reduce heat to medium and simmer until thickened, about 5 minutes. Stir in meat, peas and carrots, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper; cook 2 minutes. Divide mixture among prepared dishes. Spread top with butternut squash. Place on a large baking sheet.
Bake, uncovered, for 15 to 20 minutes or until top is lightly browned. If desired, top with shaved Parmesan and sliced green onion. Makes 6 servings.