Preheat the oven to 200°C or Gas Mark 6. Wash the lentils in cold water, until the water is no longer milky, then leave to dry. Warm the oil in a wide, shallow ovenproof pan over a medium heat, add the onion and cook until soft and pale, about 10 minutes. Stir in the ginger, garlic and cumin seeds. Continue cooking for 2 minutes. Meanwhile, crack open the cardamom pods, take out the seeds, grind to coarse powder, then stir into the onion together with the turmeric and chili flakes. Grind some pepper and add half a teaspoon of salt, stirring for a minute or two. Add the tomatoes, lentils, stock and coconut milk then bring to a simmer for a few minutes until everything has come together. While the lentils simmer, cut the sweet potatoes into long discs about 5mm thickness. Season each piece with salt and pepper, then place on top of the lentils to form a lid, brush the top with