In a large pot melt the butter over medium heat. Add the onions and sauté for about 4 minutes, until tender. Add the rutabagas and parsnips and stir, then pour in the broth, raise the heat to high and bring to a simmer. Lower the heat to medium and simmer, covered, for about 25 minutes until the vegetables are tender.
Using a slotted spoon, carefully transfer the vegetables to a food processor or blender. Pour in about 1/2 cup of the remaining cooking broth and puree with the sage leaves and honey until smooth. Season with salt and pepper. Add more liquid until you've reached your desired consistency, as thick or thin as you like (you may not use all of the broth). Serve hot with the onions or scallions sprinkled on top.