Preheat oven to 425°. Cut each squash in half lengthwise; discard seeds and membranes. Place squash, cut sides down, on a jelly-roll pan coated with cooking spray.
Combine onion and oil, tossing to coat. Spread onion mixture onto pan around squash. Bake at 425° for 45 minutes or until squash and onion are tender. Cool slightly. Scoop out squash pulp from skins; discard skins. Place onion and squash pulp in a Dutch oven. Stir in broth and the next 5 ingredients (through pepper); bring to a boil. Reduce heat; simmer 5 minutes. Place half of squash mixture in a blender. Remove center piece of blender lid to allow steam to escape; secure blender lid on blender. Place a clean towel over opening in blender lid to avoid splatters. Blend until smooth. Pour into a large bowl. Repeat procedure with remaining squash mixture. Return pureed mixture to pan; stir in half-and-half. Cook over medium heat 5 minutes or until thoroughly heated. Garnish with thyme, if desired. Makes 10 servings (1 cup each).