Boil the sweet potatoes in a generous amount of lightly salted water until tender. Meanwhile, preheat the oven to 400 degrees F. Place the halibut in a non-stick or parchment paper-lined baking dish. Drizzle with the lemon juice and 1 1/2 tablespoons of the butter; sprinkle with salt and pepper. Bake for 12 to 15 minutes, or until cooked through.
When tender, drain the sweet potatoes well and mash thoroughly. Whip in the buttermilk, maple syrup, and remaining melted butter; season with salt, pepper and nutmeg. Mound an equal amount of the mashed sweet potatoes onto four dinner plates. Top each mound with a piece of the halibut, spoon the pan juices over the fish, sprinkle with parsley, and serve.