Tip the rice into a small pan. Pour in 400ml boiling water from the kettle. Add a pinch of salt. Cover. Bring to the boil. Turn the heat right down. Simmer for 20-25 mins. Take off the heat. Steam, lid on, for 2-4 mins till the rice is cooked.
Meanwhile, trim the lemongrass and strip off the tough outer leaves. Add these to the rice. Slice the tender middle finely. Trim and cut the courgette into 2 cm cubes. Rinse the tomatoes and cut into wedges. Scrub the sweet potato and chop it in to small chunks.
Put the turmeric, curry paste, lemongrass and 1 tbsp oil in a wok and stir on a low heat for 1 min till sizzling. Add the courgette and fry for 2 mins, stirring. Stir in the coconut milk, add the tomatoes and bring to a simmer.
Add the potatoes. Turn the heat down low. Pop on the lid. Simmer for 15 mins, till the potatoes are nearly tender. Trim the beans and halve them.
Add the beans to the wok and stir in. Add a tiny splash of hot water if you think it needs to be saucier. Pop the lid back on. Simmer for another 2-3 mins, till the veg are tender but still with a bit of crunch.
Quarter the lime. Heat a small pan with ½ tbsp oil. Add the mustard seeds and fry for 1 min or till they start to pop. Tip over the curry. Serve the rice and curry with the lime wedges.