Combine the first 8 ingredients in a large bowl. Sprinkle with 1/8 teaspoon salt; toss well. Place the vegetable mixture in a single layer on a jelly-roll pan. Bake at 400 degrees for 30 minutes or until tender, stirring once.
Combine the vegetables, stock, and the remaining 1/4 teaspoon salt in a large saucepan; bring to a boil. Reduce the heat; simmer for 5 minutes.
Place half the vegetable mixture into a blender. Remove the center piece of the blender lid to allow steam to escape; secure the blender lid on the blender. Place a clean paper towel over the opening in the blender lid and process until smooth. Pour into a large bowl. Repeat the procedure with the remaining vegetable mixture. Stir in the butter. Top with rosemary if desired.