Step 1Put lemon juice, egg and salt in a blender. Slowly start blending the ingredients, adding oil little by little, until the mayonnaise is thick and you have added all the oil.
Step 2Add the chipotle chile and blend in. Makes three-fourths cup. You will have some left after making the tostadas.
Step 1Heat the oil in a deep skillet to 350 degrees. Fry each tortilla until crisp, about 2 minutes. Drain between paper towels. Set aside.
Step 2Slice the leeks (white part only) into one-fourth-inch-wide julienne. Heat the olive oil in a skillet over very low heat. Add the leeks, sprinkle with a little salt and cook until soft but not browned, about 5 to 7 minutes.
Step 3Marinate the tuna slices in the soy sauce and lemon juice for 2 minutes. Drain.
Step 4Spread 1 1/2 teaspoons chipotle mayonnaise on each tostada. Divide the tuna among the tostadas. Top the tuna with the leeks and add a slice of avocado to each tostada.