Contramar-style tuna tostadas with chipotle mayonnaise, avocado and crispy leeks

Stand the tuna on its edge and cut into two one-inch slabs, then slice about an eighth of an inch thick, drawing the knife in one direction only from the heel.

Stir together a small amount of soy sauce and orange juice; dip the tuna slices on both sides and shingle them out.

Heat a pot of about an inch of oil toward 350 to 360 degrees.

Slice the white part of the leek crosswise about an eighth of an inch thick, separate the rings in a bowl of water, then dry thoroughly on paper towels; doubled-up rings will not fry crisp.

Fry the leeks in two batches starting at 360 degrees in a deep pan, stirring, about two minutes until they start to brown; the oil drops to about 300, which gives a gentler fry. Drain on a paper towel lined tray; they crisp as they cool.

Cut small rounds from the thin dry tortillas with a mold, shaving the edges round.

Fry the rounds at about 360 degrees until the bubbling slows; they bubble but do not puff.

Finely chop one large chipotle, seeds and all, and stir into a little mayonnaise to taste.

Smear about a tablespoon of chipotle mayo on each tostada, top with a couple of tuna slices and a couple of thin slices of avocado.

Salt the crispy leeks and put them on top; serve with a lime wedge alongside.

For sashimi-quality tuna, go to a fishmonger or use flash-frozen blocks from the frozen fish section, defrosted in the refrigerator overnight.