Heat the oven to 180°C. Peel and dice the sweet potato into 2cm pieces, then put on a baking tray and drizzle with half the oil, salt and pepper. Pop into the oven for 20 mins.
Peel and finely chop the onion, then peel and chop the ginger and garlic. Remove the outer layers of the lemongrass, bash with the back of a knife and chop into two pieces.
Heat the remaining oil in a large pot over a medium heat and add the onion, garlic, ginger and lemongrass. Cook for 5-10 mins until soft and slightly golden. Add the cumin seeds and fry for 30 seconds, then add the curry paste and cook for 2-3 mins until fragrant.
Add the coconut milk and stock. Rinse the lentils, then add them too and stir well. Bring to a simmer and cook for 15 mins. Once the dal is cooked, add back in the cooked sweet potato and squeeze in the juice of the limes to taste.
