Put the oil in a large, high-sided pan over a medium heat. Add the onion and sauté for 5 minutes until starting to soften. Tip in the garlic, chilli flakes, garam masala, coriander, cumin and turmeric. Fry for 2-3 minutes, stirring now and again, until aromatic.
Add the sweet potatoes, lentils, stock and sea salt. Bring to the boil and simmer for 30-35 minutes, stirring often, until the sweet potatoes and lentils are cooked through; season.
Meanwhile, put the rice in a large pan and cover with twice its volume of salted water (about 500ml). Bring to the boil then reduce the heat to low, cover the pan and leave to simmer gently for 12 minutes, resisting the temptation to peek inside during cooking. Once the liquid has been absorbed, remove from the heat, leaving the lid on to allow the rice to steam for a further 5 minutes, then fluff up with a fork.
Stir the lemon juice into the curry and season. Serve with the rice, the yogurt stirred through and the chives scattered over.