Mix the mustard, onion, cilantro, and tuna
Heat the tortillas on a comal (griddle) to soften them
Fill the tortillas with the tuna mixture.
Place on a hot griddle and keep the heat to medium-low.
As they are toasted, flip them.
It will take around 5 to 6 minutes to toast each side, you have to keep an eye on them so they don't burn.