Cut the cucumber in half lengthways and scrape out the seeds with a teaspoon. Grate the flesh with the coarse side of the grater, sprinkle with the salt and leave to drain in a colander for 30 minutes. Dissolve the gelatine in the hot water and leave to go tepid.
Whisk the double cream to soft peaks, then fold in the tepid gelatine and then the ricotta. Squeeze the cucumber to remove any excess liquid, then stir into the cream mixture. Add the white wine vinegar to taste, remembering that this is served chilled and flavours can be dulled. Season with the pepper and chives.
Spoon into an oiled 1 litre mold and refrigerate overnight. Turn out the next day by dipping the mold briefly in hot water to loosen the mousse. Serve in individual dishes with chopped cucumber, olive, peppers and onions and garnish with mint leaves.