Bash the cumin seeds a little in a pestle and mortar to release their aroma. Mix together the lime juice, sugar, rice vinegar and cumin seeds in a small bowl. In a larger bowl, mix the remaining salsa ingredients, pour over the lime juice mixture and toss everything together.
To make the chipotle mayo, mix together mayonnaise, chipotle sauce and lime juice.
Heat an inch of oil in a saucepan over a medium heat until hot. Fry the corn tortillas one at a time until golden brown on each side and puffed. Place on a tray lined with a paper towel to absorb any oil.
Top the tostadas with salsa, prawns and avocado. Drizzle with chipotle mayonnaise to serve.