In a preheated pan, heat cooking oil.
Add the onion and garlic. Sauté until fragrant.
Add the tomatoes. Continue to sauté until tomatoes are soft.
Add the 2 cans of Century tuna including the oil. Stir and cook for 3 minutes.
Add water then bring to a boil.
Add the beaten egg. Stir and cook until it starts to curdle.
Add the ground black pepper, fish sauce, and onion leeks. Stir and cook for another 2 minutes.
Remove from heat. Transfer to a serving bowl.
Serve while hot and enjoy your Century tuna sarciado!