Fill the bottom of a large pot with about 3 inches of water, and insert a steamer basket. Bring the pot to a boil, add the sweet potato, cover the pot, and steam for about 7 minutes, until the potato slices are just fork tender. (You don't want the sweet potato to be too soft or it will become a mushy ball when mixed together.)

In a large wok over medium heat, warm the oil, and sauté the mustard seeds for a minute until they start to pop. Add the ginger, garlic, curry powder, turmeric, and black pepper. Stir in the onions with a pinch of the salt, and sauté for about 5 minutes until the onions are soft and translucent. Stir in the nutritional yeast and the lime juice.

Add the steamed sweet potato, cilantro, and the remaining salt, and use two wooden spoons to gently toss until the sweet potato is evenly coated and warmed through. Season with additional salt, to taste.