ROAST: Preheat the oven to 425 F / 220 C. Peel the sweet potatoes and chop them into even, bite sized pieces. Toss them with the fat of your choice, to coat, then spread them out in a single layer on a half sheet. Roast until tender, about 20 minutes, turning the sweet potato pieces over half way through cooking for even browning. Once done, add the roasted sweet potatoes to a mixing bowl and allow to cool slightly.
BLEND: While the sweet potatoes are roasting, peel and finely chop the parsnips. Add them to a saucepan along with the diced onion, chicken broth and salt. Cover with a lid and bring to an even simmer, then cook until the parsnips are tender, about 15 minutes. Carefully transfer the vegetables and remaining cooking liquid to a blender. Leaving a vent for steam to escape, carefully blend the parsnip mixture until it is smooth and creamy in texture.
TOSS: Pour the parsnip cream over the roasted sweet potatoes and add the green onions, celery, radishes, fresh tarragon, dill and garlic powder. Use a spatula to gently combine all the ingredients – don’t worry if the potato salad looks like it has too much parsnip cream, as it will thicken up when it is chilled. Taste and add any extra salt, if you prefer.
SERVE: Chill for several hours, or overnight, before serving, garnished with extra green onions.