It is recommended to prepare the filling one day in advance, if this is not possible, make the filling early enough to allow it to rest and dry out.

In a medium hot pan, add a tablespoon of olive oil and sauté the chopped onion and chopped serrano chile. Add the chopped tomatoes and cook until soft, approx 5 minutes.

Drain the tuna and add it to the tomato mixture, let it cook until the water from the tomato-onion mixture evaporates, leaving a moist fish mixture.

Cook and peel the potatoes (or leave them unpeeled for a different texture), then smash them. Add the butter, cheddar cheese, queso fresco, and egg white. Mix until smooth.

Pre-heat oven to 350 F. (~180 C)

Take a large, lightly floured piece of wax paper and spread out the potato mixture on it, forming a large rectangle.

Pour the tuna mixture lengthwise on top of one half of the potato rectangle, add olives on top.

With the help of the wax paper, and starting with the tuna mixture side, roll the rectangle of potato mixture until the tuna mix is rolled up inside.

Remove from wax paper and transfer to a Pyrex dish or baking sheet. Brush the top of the roll with egg yolk and put it in the oven 15-20 minutes until golden brown.

Serve alongside a green salad.