Prick the skin of the sweet potato all over with a fork and roast whole at 200°C for 1 hour.
When cool enough, peel the skin off and scoop out the flesh, blitz until smooth. Season. This can now be warmed in a saucepan.
Peel the celeriac and cut into wedges (six or eight, depending on size).
On the widest ‘back’ of the celeriac wedges, make incisions down the back, making sure not to cut all the way through.
Drizzle with oil and honey and sprinkle with turmeric and seasoning. Toss to cover well and roast at 160°C until tender and golden. About 30 minutes.
Toast the cashews in the oven at 150°C for 10-20 minutes. Check regularly and toss to toast evenly, until they are golden.
Peel and core the parsnips and cube evenly. Place in a saucepan and cover with the milk. Place on a medium heat and simmer for about 20 minutes until the parsnips are very tender.
Drain the parsnips, reserving the milk. Blitz the parsnip until smooth, adding the reserved milk until smooth and desired consistency is achieved. It should be spreadable but not runny.
For the radishes, finely slice (either with a knife or mandolin) into rounds. Turn them on their side and slice to create thin matchsticks.
To plate up, spread the parsnip puree on the plate, adding dollops of the sweet potato puree intermittently on top. Arrange the celeriac wedges on top and sprinkle with the radishes, cashews and nigella seeds.