Preheat oven to 350 degrees.

In a cast iron or oven-proof skillet set over medium-high heat, heat oil, and add in onions to caramelize, stirring constantly for 1 minute. Turn down heat to low and cook onions until they begin to turn golden. Add in sweet potatoes and stir to coat in oil and onion mixture. Cover with lid and cook until sweet potatoes are tender, about 10 minutes. Transfer to a large bowl and set aside. Keep skillet over hot flame.

In a small bowl, whisk together eggs, salt, pepper, nutmeg, parsley, and thyme. Pour over sweet potatoes and stir to incorporate. Add remaining tablespoon olive oil to skillet and pour in sweet potato-egg mixture.

Let cook over medium heat until the edges seem to firm up, about 5 minutes, then transfer to oven to cook. Tortilla is ready when all the egg has set, about 20 minutes. Let cool slightly before flipping tortilla over onto a plate and slicing to serve. Top with Maldon salt.