Preheat oven to 350°. Wash pumpkin. Cut a 5-in. circle around stem; remove top and set aside. Remove strings and seeds from pumpkin; discard seeds or save for roasting.
Sprinkle inside of pumpkin with salt and pepper; replace top. Place on a wire rack in a 15x10x1-in. baking pan. Bake 1 to 1-1/4 hours or until tender. Remove top; let pumpkin stand on wire rack until ready to serve.
In a large saucepan, cook pasta according to package directions. Meanwhile, in a large skillet, cook sausage and onion over medium heat 4-6 minutes or until sausage is no longer pink, breaking up sausage into crumbles; drain.
Drain pasta, reserving 1/3 cup pasta water. In same saucepan, melt butter over medium heat. Stir in flour until smooth; gradually whisk in cream and stock. Bring to a boil, stirring constantly; cook and stir 2-3 minutes or until thickened. Stir in cheeses until blended.
Add spinach, roasted pepper, pasta and sausage mixture; toss to combine, adding reserved pasta water to moisten if desired. Spoon into pumpkin; serve immediately.