Preheat oven to 350°F.

In a bowl, add egg, cilantro and spices. Mix well and set aside.

Add oil to cast iron skillet on medium heat. Then add onion, serrano pepper and pinch of salt.

Once the onions turn soft, add tomatoes.

Cook for about 10 minutes, or until the tomatoes soften.

Pour the egg mixture into the skillet and quickly stir it with the onions and tomatoes. Let it sit undisturbed for about 5 minutes and then put the skillet in an oven for 5 minutes or until the top layer of the egg is cooked (I used a convection oven).