Put the oil in a wide, deep pan on a medium heat.
Once it's hot, add the cinnamon stick, curry leaves and mustard seeds, let them sizzle for 20-30 seconds, then add the garlic, ginger, chillies, lemongrass and turmeric, and cook, stirring, for two to three minutes, until fragrant.
Add the leeks and ½ tsp of salt, and cook for six to eight minutes more, until softened.
Add the potatoes and coconut milk, stir to mix, then add 250 ml water and 1 tsp of salt.
Turn up the heat, bring to a gentle boil, then cover, turn down to a simmer and leave to cook for 15 minutes, until the potatoes are tender (test with the tip of a small, sharp knife: it should slip through easily).
Take off the lid, let the curry reduce for a further five minutes, then serve with roti.
