Pre-heat oven to 180C. Dice the sweet potato into cubes, toss in a little oil and salt and bake in oven for about 20-30 minutes or until cooked and slightly crispy.
Begin with dicing the onion and mincing the garlic.
Heat 1 tbsp ghee/oil in a large pot on medium-high heat.
Add in cumin seeds, mustard seeds, curry leaves and chilli if using. Stir continuously for 2 minutes or until mustard seeds start to pop. Add in onions and a pinch of salt.
Sauté onions until tender, about 5 minutes. Add in tomato paste and spices, let this sauté for about 5 minutes stirring occasionally.
Add in lentils and water, bring to a gentle simmer turning down heat to low-medium. Let this simmer until lentils are almost cooked, about 15-20 minutes. Add in coconut cream and bring this to a simmer for a further 10 minutes, adding in baked sweet potato. Add in salt to taste. Turn off heat and squeeze in half a lime.