Peel and finely chop the onion. Warm a large pan or casserole dish over a medium heat for 2 mins, then add the onion with 2 tbsp oil. Season with a pinch of salt. Fry for 6-8 mins, stirring every so often, till the onion has softened and picked up a little colour.
While the onion fries, peel the carrots and thickly slice them on the diagonal. Break the broccoli into small florets. Chop the stalks into thick chunks to match the florets. Scrub the potatoes and chop into bite-size chunks.
Stir the carrots, broccoli and potatoes into the onion. Add 2 tbsp water. Pop a lid on the pan (or use a baking tray if you don’t have a lid) and cook for 10 mins, stirring once or twice, till the veg start to soften.
While the veg sweat, halve the pepper and scoop out the seeds and white pith. Roughly chop the pepper. Trim the top off the courgette, then halve it and slice it into ½cm thick slices. Stir them into the veg and sweat for another 5 mins.
While the veg continue to cook. roughly chop the cashews. Peel and grate or crush the garlic. Halve the chillies and scoop out the seeds and white pith (or leave them in for more heat). Finely chop them.
Stir the cashews, garlic and chillies into the veg. Add 1 tsp each ground ginger and turmeric. Drop in 6 curry leaves and stir everything to mix. Finely grate in the lemon zest and stir again.
Pour in the coconut milk. Add 300ml boiling water. Put the lid back on the pan and bring to the boil. When it’s boiling, turn the heat down and simmer for 10-15 mins till the veg are tender.
Squeeze the juice from the lemon into the curry. Taste and add a pinch more salt if you think it needs it. Ladle the curry into warm bowls to serve.