Heat the rapeseed oil in a large soup pan over a medium high heat. Add the black mustard seeds and curry leaves, and cook until the seeds begin to pop.
Add the onion, garlic, butternut squash, curry powder, chilli, cumin and cardamom. Reduce the heat and cook for about 10 minutes, stirring occasionally, until the vegetable begin to soften.
Add the stock and the turmeric and season to taste. Simmer for about 20 minutes until the vegetables are tender.
Using a hand-held or high-speed blender, blend until smooth.
Serve with a dollop of natural yogurt and a scattering of fresh coriander leaves (not pictured) if desired.
Use of red chilli: if you want a nice kick add the seeds and the chilli whole. I use a small chilli, about 1.5 inches long. If you want a more mild soup (or you're cooking for children), substitute the fresh chilli with a small pinch of chilli flakes.
Mum of three, daydream adventurer, OU Life Sciences graduate, occasional crafter, ex-pat Canadian & quite possibly Britain's most northerly food blogger.