2/3 cup (165ml) rice vinegar
2/3 cup (150g) caster sugar, plus extra 1 tsp
2 Lebanese cucumbers, peeled, seeds removed, thinly sliced
4 corn tortillas
Sunflower oil, to shallow-fry
3 eschalots, thinly sliced into rings
1 tbsp cornflour
2/3 cup (200g) whole-egg mayonnaise
2 tsp finely chopped chipotle chillies in adobo sauce, plus 1 tsp adobo sauce
1 tbsp each Dijon mustard + soy sauce
Juice of 1 lime
250g sashimi-grade tuna, thinly sliced
Black sesame seeds + coriander leaves, to serve (optional)
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.
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.
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.
Combine soy sauce, lime juice and extra 1 tsp caster sugar, then add the tuna and gently toss with
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.