Preheat the oven to 200°c/180°c fan-forced. Scrub the parsnips and potatoes (there's no need to peel them), peel the onions and finely slice everything, adding to a large casserole dish as you go.

Peel, finely slice and add the garlic, strip in most of the thyme and roughly bash most of the hazelnuts and add. Drizzle over 2 tbs of oil, add a good pinch of see salt and black pepper and toss everything together well. Arrange in a fairly even layer, pour over 300ml of water, then tightly cover with foil and roast for 1 hour 10 mins.

Remove the pan from the oven, lift back the foil and drizzle over the cream. Place the foil lid back over the top, then put a folded tea towel on top and carefully press down to compress everything. Discard the foil.

Finely grate over the cheese, scatter with the remaining hazelnuts, if using, and pick over the remaining thyme leaves. Drizzle with 1 tbs of oil, then return to the oven for a further 20 mins or until golden, bubbling and cooked through. Allow it to sit for a few minutes before serving.