Preheat oven to 350°. Cut squash into eight large pieces. Place cut side up in a 15x10x1-in. baking pan. Cut 1/4 in. off tops of onion and garlic bulbs (the end that comes to a closed point).
Place cut side up in baking pans. Brush with oil; sprinkle with thyme. Cover tightly and bake 1-1/2 to 2 hours or until vegetables are very tender. Uncover and let stand until lukewarm.
Remove peel from squash and onions; remove soft garlic from skins. In a large bowl, combine vegetables, broth and cream. Puree in small batches in a blender until smooth; transfer to a large saucepan. Add parsley, salt and pepper; heat through (do not boil). Garnish with thyme if desired. Yield: 8 servings (2 quarts).