Preheat oven to 375°F. Toss the carrots, parsnips, onion and garlic in 3 tablespoons olive oil and a pinch of salt and pepper. Spread veggies in an even layer onto a foil-lined baking sheet and roast for 25 minutes, until everything is tender. Remove and cool slightly.
Place the roasted carrots, parsnips, onion and garlic (squeezed out of their skins) and any juice that has accumulated on the bottom of the pan in a large saucepan then add in the remaining olive oil, ginger and roughly half of the vegetable stock. Using an immersion blender, purée until completely smooth. (If you do not have an immersion blender, use a conventional blender then pour into the saucepan.) Add the remaining stock and stir. Bring to a simmer then serve.