Does anybody recognize this Finnish dill sauce?
A very common fish condiment in Sweden is "dillsås", dill sauce. 


10 oz gräddfil. A light sour cream. (yoghurt?)
A few tablespoons of mayo
A generous amount of chopped dill. 
Season with salt and white pepper to taste. 


You can add some mustard and lemon if the mayo is bland.

The taste is smooth and quite neutral with the dill as the lead actor. 

A note on the sour cream. It can't be a thick, heavy consistency. It should be quite light and slightly uneven in texture (whisking makes it smooth). It is almost yoghurt like and contains about 10% fat.  I have seen suggestions that low fat sour cream can be quite similar. I get the impression IKEA stocks it globally.

It should look something like 


Image from http://chezsofia.bloggsida.se/2009/08/dillsas-till-saftiga-grillspett/.