Prepare potatoes: Cook potatoes in advance, peel them, and mash them well using a glass or similar tool. Let the mashed potatoes cool slightly.
Prepare herbs and onion: Chop cilantro and half a red onion into small pieces.
Mix pancake ingredients: In a bowl, combine mashed potatoes, chopped cilantro and onion, eggs, canned tuna, crushed saltine crackers, and salt. Mix everything well by hand. Add the second egg if the mixture seems too dry.
Form and fill pancakes: Shape the mixture into small pancakes. Place two small pieces of cheese inside each pancake and cover them.
Fry pancakes: Heat oil in a pan. Fry the pancakes on one side until golden, then flip and fry the other side until cooked through and browned.
Prepare flag sauce: Chop cilantro, serrano peppers (4), tomatoes (4), and an onion. Mix all chopped ingredients and add salt to taste.
Serve: Serve the tuna potato pancakes hot with the flag sauce on top. Optionally, serve with rice.