For the pickled cucumber, peel alternate lengths of the cucumber, so that it has a stripy skin. Cut the cucumber in half lengthways, scoop out the seeds with a spoon and slice the flesh thinly on the diagonal.
Place the cucumber slices into a bowl with the salt and mix well. Set aside for 30 minutes. When the cucumber has marinated, heat the rice wine vinegar and sugar in a non-reactive saucepan until the sugar has dissolved.
Rinse the salt from the cucumber slices, transfer them to a clean tea towel and squeeze any excess water from the cucumber.
Add the cucumber slices to the vinegar mixture and stir until well combined. Stir in the chili and coriander seeds. Cover the saucepan with cling film and set aside until needed.
Meanwhile, for the cucumber soup, heat the oil in a saucepan over a medium heat. Fry the ginger and chili for 2-3 minutes, or until softened.
Add the cucumber and cook for a further 2-3 minutes. Add the stock and bring the mixture to the boil. Reduce the heat until the mixture is simmering and simmer for 2-3 minutes.
Transfer the soup to a food processor and blend until smooth, then return to the saucepan. Season, to taste, with salt, freshly ground black pepper and lemon juice.
Just before serving, stir the mint and coriander into the pickled cucumbers and season, to taste, with salt and freshly ground black pepper. To serve, divide the cucumber soup equally among four serving bowls. Drizzle over a little olive oil. Garnish with the pickled cucumber.