Fry the leeks and tostadas. If the tortillas are at all moist, lay them out in a single layer to dry out. Scoop the leeks into a bowl, cover with cold tap water and work them between your fingers to separate the rings. Drain well and spread onto paper towels. Thoroughly pat dry. Heat the oil to 360 degrees. In two batches, fry the leeks until they turn a rich golden and are beginning to crisp, about 3 minutes (they will not be completely crisp). Use a slotted spoon or skimmer spider to remove to a paper towel-lined baking sheet. When all are fried, spread out the leeks and sprinkle with salt. One or 2 at a time, fry the tortillas until crisp, turning regularly, about 1 minute (the bubbling will have nearly stopped). Drain on paper towels.
Marinate the tuna. In flat dish (an 8x8-inch baking dish is perfect), mix together the soy and orange juice. Slice the tuna into thin slices (about 1/8 inch) and lay into the pan in a shingled single layer, flipping each slice to season both sides with the marinade. Let stand about 10 minutes.
Make the chipotle mayonnaise. In a small bowl, combine the mayonnaise and chopped chipotle.
Assemble the tostadas. Spread about a tablespoon on each tostada. To finish each tostada, lift a couple slices of tuna from the marinade, lay them over the mayo, top with a generous sprinkling of the crispy leeks and a couple of avocado slices. Serve with lime wedges.