Peel a few thin, vertical slices of cucumber and set aside on a paper towel to absorb the liquids. Roughly grate the rest of the cucumber and squeeze out the excess juice. Season with salt and place on a paper towel to absorb the remaining juice while you mix the rest of the ingredients.
In a large bowl mix the yogurt with the onion, the mint, the ground chilli, cumin, coriander a little bit of sugar and salt to taste. Add the grated cucumber and mix well.
Serve cold, garnished with the cucumber peels and sprinkle with the lemon rind and extra chilli flakes if desired.