Heat the oil in a casserole dish or large, heavy saucepan over a medium heat. Add the mustard seeds and fry briefly until they begin to pop. Add the onion, ginger, garlic, cinnamon stick and cardamom pods and fry gently for 5 minutes, stirring often, until the onion has softened and the mixture is fragrant.
Meanwhile, mix together the coriander, cumin, turmeric and chilli powder with 2–3 tablespoons of water to form a spice paste.
Add the spice paste to the onion mixture. Fry, stirring, for 1 minute. Add the coconut milk and stir while bringing to the boil. Add the mushrooms and simmer for 5 minutes. Mix in the spinach and cook briefly until just wilted. Season with salt and pepper and serve at once with rice.
Serve with perfumed basmati rice, naan or paratha flatbreads for a delicious taste of the tropics