Method
Heat the oven to 200°C/180°C fan/400°F/gas mark 6.
Scatter the parsnip chunks in a roasting tin. Chuck in the garlic bulb halves, along with the chestnuts, sage, onion and olive oil and season with plenty of salt and pepper. Cover the roasting tin tightly with foil and place it in the oven. Cook the parsnips, shaking the tin occasionally, for about 1 hour, until soft and caramelised. Then, carefully remove the foil (it will be steamy) and pour in the stock. Return the tray to the oven for a further 30 minutes. (At this point I like to remove a few broken chestnuts, onion pieces and half-garlic cloves to garnish the soup before serving, but it’s up to you.)
Ladle everything except the garlic out of the tray and into the jug of a blender. Squeeze the roasted flesh out of the papery skins of the garlic halves and add it to the jug. Whiz up the soup until it’s lovely and smooth.
Pour the soup into a pan. Pop it on a low heat and bring to the simmer. Cook gently for 10 minutes, taste and adjust the seasoning with salt and pepper, then ladle it into warmed bowls. Serve with the reserved chestnuts, onion and garlic scattered over the top (if using), as well as a trickle of olive oil and a grinding of salt and pepper.