Saute the onion on high heat in a deep saucepan, adding the potatoes and carrot after a few minutes.
Meanwhile, toast the coriander, cumin and mustard seeds in a small dry frypan until the mustard seeds start to pop. Grind them in a mortar and pestle or spice grinder then add to the vegetables along with the turmeric and chillies.
Adjust the heat to medium low and add the coconut milk, ginger and garlic. Add salt as desired and let simmer until the potatoes are done. Add the capsicum and peas and simmer one more minute, then just before serving add the lemon juice and coriander.