Put the yoghurt in a mixing bowl and beat it with a fork until very smooth.
Top and tail the cucumbers and coarsely grate them into the same bowl. Finely grate the garlic and add it to the yoghurt mixture with half a teaspoon of salt, all but a scattering of the fresh mint and the dried mint, if using.
Pour over some olive oil in a swirl, top with the last of the fresh mint and serve with the koftas above.