My tuna tostada is composed of layers: a smoky chipotle mayonnaise spread over the shell, topped with slices of silky tuna, briefly marinated in ponzu, capped with a scattering of crispy fried leeks and a single perfect slice of avocado. The success of this dish depends on choosing top-quality fish, of course. At Contramar, we only serve it when sustainable tuna is available, so it’s not always on the menu. In San Francisco, I’ve substituted a locally farmed trout, although the rest of the recipe is unchanged. You can also make these with hamachi, kampachi, or albacore. But in my opinion, raw tuna and trout are especially beautiful for their deep pink color. Whatever fish you use, make sure that it’s sashimi-grade and slice it as thinly as possible against the grain.
For the fried tostada shell, it’s ideal to have stale corn tortillas because fresh tortillas curl up in the fryer. In Mexico, you always see people drying out their old tortillas, either to feed to the pigs and chickens or to use in chilaquiles and tostadas. If you know you are going to want to make these tostadas for a party, then plan ahead and leave your tortillas out for a day or two first. Place them in a single layer on a baking sheet with another baking sheet on top of it to keep them flat as they dry out. If you go the extra step of first making tortillas from scratch solely for the purpose of making tostadas, be sure that they’re quite thin: no more than 1⁄8 inch / 3mm thick. A thicker tortilla will be difficult to get crisp in the fryer.
This is finger food, so I like my tostadas to be fairly small. To make smaller (than 6-inch/ 15cm) ones, simply, cut your tortillas in half and then fry them.