1. Preheat oven to 400°.
2. Combine first 8 ingredients in a large bowl. Sprinkle with 1/8 teaspoon salt; toss well. Place vegetable mixture in a single layer on a jelly-roll pan. Bake at 400° for 30 minutes or until tender, stirring once.
3. Combine vegetables, stock, and remaining 1/4 teaspoon salt in a large saucepan; bring to a boil. Reduce heat; simmer 5 minutes. Place half of vegetable 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). Process until smooth. Pour into a large bowl. Repeat procedure with remaining vegetable mixture. Stir in butter. Top with rosemary, if desired.