Sauté the onions on a low heat in 2 tablespoons of olive oil until they are soft and tender.
Peel and thinly slice the sweet potato. Fry them in a frying pan until tender in 2/3 tablespoons of olive oil.
Whisk together the eggs, onions, cheese and spices.
Evenly spread the cooked sweet potatoes in the pan and pour the egg mixture over them. Cover the pan with a lid and let it cook on a low heat for 10 minutes.
After 10 minutes, flip the tortilla and cook for another 2 minutes. I use the lid of a pan for flipping. You can serve the tortilla immediately or keep it for later (great for parties or lunch!). It is also delicious when served cold.