Peel the cucumber, cut in half lengthwise and remove the seeds. Cut the halves into slices. Cut the tomatoes in half diagonally, remove the seeds and dice. Remove the skin from the onion and chop coarsely.
In a baking bowl, mix the yogurt with the cumin and a tiny bit of salt until smooth. Add the cucumber, tomatoes and onion.
In a small pan, heat the oil. Pour in the mustard seeds and toast briefly over medium heat with the lid closed (the seeds will pop!). Immediately add to the lettuce and mix everything together thoroughly.
Let sit for at least half an hour before serving.
Tip: Use creamy natural yogurt for an even finer result!