Preheat the oven to 350 degrees F.
Roast the whole sweet potatoes on a baking sheet until tender, about 1 hour. Let cool for 10 minutes, then halve lengthwise and scoop out the flesh into a bowl. Mash to an even consistency. Season with salt and pepper.
Heat the clarified butter in a small saucepan over medium-high heat until shimmering, about 2 minutes. Add the coriander seeds, cumin seeds, mustard seeds, turmeric and curry leaves if using and shake the pan over the heat, taking care as the spices sizzle and pop. Remove from the heat once the spices become fragrant, about 1 minute. Pour over the sweet potatoes and fold in the cilantro and lime juice. Serve hot.