Dice the onion very finely. Put the mayonnaise, oil, sugar, pepper, chives, salt, dill, tarragon and condensed milk in a bowl and stir thoroughly.
Only when the oil is no longer recognizable as such, add a more or less powerful squirt (depending on how sour you like it) lemon juice and stir in. The condensed milk then coagulates and ensures that the salad sauce thickens and stands.
Halve the cucumber and peel from the cut towards the ends. Cut into wafer-thin slices and add to the sauce. Stir well and let it steep for a few minutes.