Preheat the oven to 400 degrees F. Pierce the sweet potatoes with a paring knife and bake for 50 to 60 minutes, until tender. About 20 minutes before the potatoes are done, bring a medium pot of salted water to a boil. Add the rutabaga and boil for 15 to 20 minutes, until tender. Drain and put into the bowl of a food processor along with the cream, butter, brown sugar or maple syrup, and parsley. Puree until smooth. Transfer to a mixing bowl. Halve the baked sweet potatoes and scoop out the flesh. Mash together the sweet potatoes and rutabaga puree, adding salt and pepper to taste.