Peel and finely chop the onion, cut the lettuce into bite-sized pieces and the radishes into sticks.
Mix water, oil and vinegar of a bowl.
Add sugar, onion, mustard and salt. Mix everything together well.
Add lettuce and radishes and let sit for a while, stirring frequently.
Arrange the salad and serve.
Preparation Tip:
The salad can be supplemented and varied as desired with other vegetables or ingredients.