Heat oven to 450 degrees F. Coat roasting pan with nonstick cooking spray.
Halve squash lengthwise; remove seeds. Cut halves in half crosswise. Place squash, potato, onion and garlic in pan. Cover tightly with foil.
Bake in 450 degrees F oven 55 to 60 minutes, until tender. (To microwave, place vegetables in microwave-safe 13 x 9 x 2-inch baking dish. Cover tightly with plastic wrap. Microwave at 100 per cent for 20 minutes. Carefully uncover.) When cool enough to handle, scoop flesh from squash and potatoes; remove skins from onions and garlic. Place cooked vegetables in processor. Whirl until smooth.
Combine puree, broth, sage, salt and pepper sauce in saucepan. Simmer 5 minutes. (Can be made up to this point a day ahead; refrigerate.)
Chop chives in food processor. Add sour cream. Whirl until blended. (Can be made 3 hours ahead; refrigerate.)
To serve, heat soup. Stir in half-and-half; gently heat through. Spoon into bowls. Drizzle Chive Cream over top. Garnish with chives if desired. Makes 8 servings.