In a bowl, combine soy sauce, lime juice, and ginger. Marinate the tuna in this, turning ocassionally to fully impregnate.
For the spicy mayo: ideally, you'd use Pequin pepper powder, but that's not always easy to find, so you can substitute with Cayenne pepper, as we did here. Add the powder to the mayonnaise and mix well.
Next, cut the leek in half and slice it into half-moons, as thinly as possible.
In a skillet, heat frying oil on high and fry the leeks until golden brown. Be careful not to let them darken too much, as they will become bitter. Remove from heat and let cool on paper towels to absorb excess oil.
Using the same skillet (caution: very hot), heat the olive oil and sear the tuna. No more than one minute per side! Then, transfer to a cutting board and slice it as thinly as possible.
Spread the spicy mayo on each tostada and distribute the tuna slices on them.
Add 1/4 of the avocado on each tostada and top with a generous pinch of fried leek. Voil! Enjoy!
