Peel the cucumber and grate thin slices in a bowl. Salt well and let it steep, squeeze it out again and again and pour off the water, add salt if necessary.
Put the vinegar in a bowl, add a little salt and stir until the salt melts. Add the onions and garlic, let it steep a little and add the oil. Season to taste with a little pepper and mix well.
Squeeze the cucumber well and mix with the sauce. Sprinkle with some fresh dill.