Heat the oil in a pan over a medium-high heat and fry the onion for 5-6 minutes, until translucent. Add the coriander, cumin and fennel seeds and curry leaves, then cook for 4-5 minutes until the leaves are bright green.
Add the water and sliced green beans, along with the turmeric and green chillies. Cook for 5-7 minutes, until the beans are bright green.
Stir in the coconut milk and add a little salt. Bring to a gentle boil, then turn down and simmer for 3-4 minutes, until the beans are tender and just cooked through. Switch off the heat.
In a small frying pan, make the temper. Heat the oil over a medium-high heat, and when sizzling add the curry leaves, fenugreek seeds and black pepper. After about 30 seconds or 1 minute, when the curry leaves are crispy, pour the whole lot on to the curry and stir through. Finish with a squeeze of lime.