It sounds to me just like a dressing my Aunt used to make in Germany I love that dressing.I did get a recipe from a german helicopter pilot who works in the states and he got his mother in Germany to send this .
The recipe he sent has no amounts so its just ingredients.
1 small onion diced
a pinch of sugar and salt and pepper
either some sweet cream or if consistancy is good some 10% condensed milk I think he means evaporated milk.Condensed milk in the states is really,really sweet.
Mix oil and vinegar together stir in cream add sugar,salt and pepper add onion stir well and put in fridge1-2 hours
You can substitute milk/cream with mayonaise which will make it firmer but she says if you get right milk it will just wonderful.My friend said his grandmother always just the condensed milk.So since this recipe is from Germans maybe you can find the kind of condensed milk they are talking about.
It sounds similar to what my aunt would make was so delicious I have not been able to recreate her recipe exactly she didn't meashure either .
I have not tried to make the above recipe.If you try to make above recipe please let me know how it turns out.