Sift the flour into a bowl with 1 tsp baking powder. Add 1 tbsp oil, half the pot of yogurt and 1 tbsp warm water. Stir to make a dough, adding 1-2 tbsp more water if the dough needs it to come together, then knead for 2 mins till soft and smooth. Pop back in the bowl, cover with a damp tea towel and leave to rest while you make the curry.
Coarsely grate the courgette. Tip into a sieve, toss with a pinch of salt and set over a bowl to drain. Tip the cashews into a pan. Toast over a medium heat for 3-4 mins till golden. Tip into a small bowl and turn the heat off.
Peel and slice the carrots. Separate the broccoli into small florets and slice the stalk thinly. Peel and halve the onion. Roughly chop 1 half. Thickly slice the other half.
Peel and grate the ginger and garlic. Halve the chilli. Remove the seeds if you prefer. Chop the coriander stalks (keep the leaves for later). Put the ginger, garlic, chopped onion, chilli and coriander stalks into a small food processor or blender. Whizz to make a rough curry paste. No processor? See our tip below.
Heat a pan with ½ tbsp oil and add the sliced onion and curry paste. Cook for 5 mins till the onion is just softened. Stir in 1 tsp curry powder, the coconut milk, and carrots. Bring to the boil, then cover and simmer for 5 mins. Add the broccoli, cover and simmer for 6-8 mins or till all the veg are just cooked. Add a splash of water if needed.
Squeeze any excess water from the courgette using your hands. Shred the sorrel leaves. Zest half the lemon. Mix together in a bowl with the remaining yogurt to make a raita. Cut the rest of the lemon into wedges.
Heat a frying pan. Divide the dough into 4 pieces and press into flat naan breads. Cook each naan for a few mins on each side till golden brown. Serve the curry in bowls, topped with the toasted cashews, coriander leaves and a lemon wedge each. Serve the naans and raita on the side.