Scrub the beetroot and slice into small, bite-size chunks (no need to peel, although you can if you prefer). Peel the onion and thinly slice it. Roughly chop the tomatoes.
Make the curry paste: peel the garlic cloves and drop into a pestle and mortar. Crush well. Halve the chilli, flicking out any seeds and membrane for less heat, and finely chop. Slide the chilli into the pestle and mortar and spoon in 1 tsp each ground cumin and coriander. Add a pinch of salt and pepper and 1 tbsp oil and stir together. No pestle and mortar? Chop the garlic and chilli as finely as you can, then stir everything together.
Pour 1 tbsp oil into a pan and bring to a medium-high heat. Slide in the sliced onion and fry for 4-5 mins, stirring occasionally, till brown at the edges. Reduce the heat slightly, then stir in the beetroot and curry paste and continue to cook for 2-3 mins.
Add the chopped tomatoes, curry leaves and coconut milk to the pan and stir well to combine. Bring to the boil, then partly cover with a lid and reduce to a simmer. Cook for 30 mins, or till the beetroot is tender. Add a splash of water if the curry starts looking too dry to you.
Meanwhile, tip the rice into a sieve and give it a rinse. Tip into a small pan and add 300ml boiling water and a pinch of salt. Pop on a lid and bring to the boil, then turn the heat right down and very gently simmer for 25 mins, till all the water has been absorbed. Take the rice off the heat and leave it to steam in the pan with the lid on while you finish the rest of the dish.
While the curry cooks, shred the cavolo nero. As soon as the beetroot is tender, stir in the greens and cook for 2-3 mins, till wilted. Grate in the lime zest, and squeeze in half of the juice.
Serve the vibrant beetroot curry next to heaps of brown basmati rice and with wedges of the remaining lime half.