Preheat the oven to broil.
Add the sweet potatoes chunks and 1 tablespoon of the salt to a large pot, and cover with cold water. Bring the water to a boil, then reduce the heat, and allow the potatoes to simmer for 20 minutes or until they become soft.
While the potatoes simmer, melt 2 tablespoons of butter in a large saucepan. Cook the onions in melted butter for 10 minutes. Add the parsnips, celery, fennel, Brussels sprouts, parsley, black pepper, and remaining salt to the large saucepan and mix well.
Pour the vegetable broth over vegetables and increase the heat. Simmer for 15-20 minutes or until the vegetables are soft. Mix the spinach or Swiss chard with the cooked vegetables, and stir the vegetable mixture until the leaves wilt. Remove the vegetables from the saucepan using a slotted spoon to drain, and transfer to a casserole or a 9-inch pie plate.
Drain the potatoes using a colander and return them to pot or large mixing bowl. Add the additional 4 tablespoons of butter and mash until smooth. Top the vegetables with the mashed potatoes, spreading evenly.
Place the pie into the oven and broil for 5 minutes or until lightly brown. Sprinkle with the nutmeg prior to serving.
