1Emulsify the egg, vinegar, and salt in a bowl by whisking vigorously, then gradually add the sunflower oil and tuna oil until thickened.
2Cut the completely cold potatoes into thin slices and finely grate the hard-boiled eggs.
3Mix the potatoes, eggs, tuna, pickles, and mayonnaise until creamy but firm.
4Spoon the mixture onto a rosquilla. Serve plain (bicicleta), topped with salt-cured anchovy (marinera), vinegar-cured anchovy (marinero), both (matrimonio), or smoked salmon (sirena).