Method

• 1.
  Combine vinegar and sugar in a small saucepan over medium heat, stirring to dissolve sugar. Bring to a simmer, then set aside to cool slightly. Add the cucumber, then chill for 30 minutes to pickle.
• 2.
  Using a 7.5 cm scone cutter, cut tortillas into 12 rounds. Heat 1cm oil in a fry pan over medium-high heat, then shallow-fry tortillas, turning, for 1-2 minutes or until crisp and golden. Drain on paper towel. Return pan with oil to medium-high heat. Place eschalot in a bowl with cornflour and toss to coat, then shallow-fry, in batches, for 1-2 minutes or until golden and crisp. Drain on paper towel.
• 3.
  Combine the mayonnaise and chipotle chilli in a bowl and season. In a separate bowl, combine Dijon with reserved adobo sauce. Set aside until ready to serve.

• 4.
  Combine soy sauce, lime juice and extra 1 tsp caster sugar, then add the tuna and gently toss with the dressing.
• 5.
  To serve, spread the chipotle mayonnaise over each tostada, then divide the drained pickled cucumber among the tostadas and top each with a piece of drained tuna. Dollop over a little mustard adobo sauce and serve scattered with eschalot, black sesame seeds and coriander, if using.