Preheat the oven to 180ºc and cut the very top off the head of garlic. Place on a piece of foil and drizzle with olive oil and sprinkle with some salt. Wrap in the foil and roast in the oven for about 30-40 minutes until the garlic is pale golden and very soft.
While the garlic is roasting, heat the vegetable oil in a large heavy sauce pan over medium heat. Once the oil is hot, add in the sliced onions and sweat them until they’re soft and translucent. Mix in the spices and the remaining thyme leaves and stir well.
Add the chopped parsnips to the pot and mix well to coat the parsnip in the spices. Pour the hot stock into the pan and bring to a boil. Turn the heat down and continue to simmer uncooked for about 15-20 minutes until the parsnips are soft.
Remove the roasted garlic from the oven and use a small knife or a toothpick to pull the cloves out of their sleeves and add to the pot. Using an immersion blender, blend the soup until it’s creamy and smooth. It will be very very thick. Add in the lemon juice and the milk if using. Taste and adjust the seasoning.
For serving you can make croutons by frying some bread in some olive oil and adding in the chopped hazelnuts right at the end, or you can use shop-bought, or skip it altogether.