Stir the chopped preserved lemon into the mayonnaise. Thin out with a little warm water, then cover and chill until needed.
- Heat olive oil and 20g butter over medium-low heat. Add onion and garlic, and cook, stirring, for 2-3 minutes until soft but not coloured. Add rice and cook for 1 minute, stirring to coat grains, then add wine and allow to bubble for 1-2 minutes until almost evaporated. Add stock, a ladleful at a time, stirring and making sure each is absorbed before adding the next, until rice is al dente. This will take about 20 minutes. With the final ladleful, stir in prawn meat, zest, cheese, parsley and remaining butter. Season. Spread on a tray and allow to cool, then chill for 30 minutes.
- Place flour, egg and crumbs in separate bowls. Roll 2 heaped teaspoons risotto into a bite-sized ball. Repeat to make 60 balls. Roll first in flour, then egg, then coat in crumbs. Chill for at least 1 hour until firm. (You can do this up to 24 hours ahead.)
- Half-fill a deep-fryer or large pan with canola oil and heat to 190°C (a cube of bread will turn golden in 30 seconds). In batches of 10, fry arancini for 2-3 minutes until golden. Drain on paper towel. Keep warm in a low oven, then serve with the lemon mayonnaise.