In a bowl, combine the prawns with the garlic, chilli, sugar, red onion and half the lime juice, then cover and chill for 2-3 hours, or overnight. (Don’t season the mixture at this point, as the salt will draw out the moisture from
Preheat the oven to Gas Mark 5, 190°C, fan 170°C. To prepare the tostadas, stamp out about 18 circles (approximately 5cm (2in) in diameter) from the tortillas using a small pastry cutter. Brush with the oil, on both sides, then transfer to a baking sheet. Season well.
Bake the tostadas for 8-10 minutes, or until crisp and lightly golden. Remove from the oven and leave to cool.
Just before serving, peel and de-stone the avocado. Put the flesh in a food processor with the remaining lime juice and blend until smooth. Season to taste.
Season the marinated prawns and toss with the chopped coriander. To assemble the tostadas, put a teaspoon of the avocado mixture on each base, then top each with a single prawn, plus a little of the chilli and onion from the marinade mixture. Garnish with a sprig of coriander, to serve.
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