Preheat the oven to 200C (180C fan). Toss the parsnip chunks in 1 tablespoon of the olive oil, spread on a roasting tray, and roast for 25-30 minutes, turning once, until tender and caramelised at the edges.
While the parsnips roast, heat the remaining oil in a large saucepan over medium heat. Add the onion and cook for 6-8 minutes until softened, then add the garlic and cumin and cook for another minute.
Add the roasted parsnips and stock to the pan. Bring to a simmer and cook for 10 minutes to let the flavours combine.
Blend the soup until completely smooth. Stir in the cream, if using, and season to taste.