Melt the butter in a heavy bottomed pot. When melted and frothy, add the saffron and cook for a mn. Peel the vegetables. Cut the carrots (or leave whole with a tiny piece of green if small, they look nicer) and parsnips in long sticks, the celery and rutabaga in small cube-like pieces.
Add the vegetables and cook for a few mns. Add the broth and bring to a simmer. Cook until the vegetables are softer but still firm. Add the cream and prolong the cooking time for 5 mns. Then away from heat, add the coarsely chopped arugula.