Peel and finely slice the onion
Put it in a plastic clip top container with the lime juice and sugar
Shake well and put in the fridge to marinate for at least half an hour - ideally overnight
Slice the cucumber lengthwise
Use a teaspoon to remove the seeds and pulp from the cucumber
Cut the cucumber into 1/4cm thick chunks
Put the cucumber pieces in a colander and sprinkle with salt. Leave to drain for half an hour or so.
Rinse off the salt and dry the cucumber in a tea towel
Drain the onion slices, reserving the lime juice
Put the cucumber and onion slices into a bowl.
Put the oil into a jar and add the lime juice to make a dressing
Pick the leaves off the mint and tear them into the dressing jar
Season the dressing well with salt and pepper, seal the jar and shake well to mix
Pour the dressing over the cucumber and onion and use your hands to mix together
This salad can be kept in the fridge for up to 24 hours before serving