1. Fill your kettle and boil it. Peel and finely chop the onion. Peel and finely grate the garlic and ginger. Halve the chilli, if using, and flick out the seeds and finely chop the chilli.
2. Trim and chop the courgettes into 1cm-thick slices. Do the same with the carrots (no need to peel – just give them a good scrub). Halve or quarter any large potatoes, so they’re all roughly the same size.
3. Place a large, heavy-based pan on a medium heat for 2 mins. When warm, pour in 2 tbsp oil and tip in 1 tbsp each cumin seeds and mustard seeds, and the curry leaves. Add a pinch of salt, then stir and fry for 2 mins till the seeds start popping. Take care not to let them burn.
4. Turn the heat down to low and add the onion to the pan. Fry with the seeds for 5 mins till just softened, stirring often.
5. After 5 mins, add the garlic, ginger and courgettes. Add 1 tbsp turmeric and the chopped chilli and stir. Turn the heat back up to medium and fry for 2 mins.
6. Add the carrots and potatoes to the pan. Pour in the coconut milk and 500ml boiling water. Add a good pinch of salt and pepper.
7. Bring the curry up to a bubble, then turn the heat to low. Cover the pan and simmer for 10 mins till the potatoes are nearly tender. While the curry is cooking, chop the flat beans into thin slices, almost shredding them.
8. When the curry has cooked for 10 mins, toss the shredded flat beans into the pan. Stir together and cook for a further 2-3 mins till the beans are al dente (soft but with a slight bite).
9. Taste the curry and add more salt and pepper if necessary. Spoon the curry into warm bowls. Scatter over a pinch of coconut flakes each. Serve with a wedge of lime to squeeze over.
10. Eat & keep^
This curry will keep covered in the fridge for up to 3 days, or freeze for up to 3 months. Simply divide into individual portions, cool and then seal and chill or freeze. Defrost the curry overnight and make sure it is piping hot when you reheat it.