Spiced Parsnip and Butternut Soup

Curried parsnip soup is a classic, invented by the late, great Jane Grigson.  The original recipe calls for 1 parsnip and I always puzzled about that, thinking it was an exiguous quantity for a whole pot.  And then I got my Riverford deliveries and realised that she meant proper parsnips, great big beasts, with the earth clinging damply to them.  Anyway, I had some to use up, along with half a butternut squash.  They both take well to spicing and, together, made a lovely sweet, silky soup that would easily serve 4 people in big bowls with lots of crusty bread or a warm buttery naan.  It is made in the usual way - 1 large onion, chopped, a large or 2 medium parsnips, peeled and chopped, and an equivalent quantity of butternut, peeled and chopped (a small specimen or half a large one).  Sweat in a tablespoon of oil and a good knob of butter until softening.  Stir in a couple of cloves of garlic, peeled and chopped and an inch or so of fresh ginger, peeled and chopped.  Add a teaspoon each of cumin seeds (ground or whole, according to your preference), a teaspoon of ground coriander and a little chilli powder (to taste - you don't want it too hot, in my opinion).  Now, just cover with veg stock (marigold, as always, is fine) and simmer for 20 minutes.  Blend, add masses of chopped fresh coriander or parsley, squeeze a little lemon in and serve with a blob of yoghurt.