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/.

Finn here. Tillikastike (dill sauce) is almost always prepared with kermaviili, a type of viili (vaguely yoghurt-like fermented milk product) which is unfortunately basically unknown outside Scandinavia. Gräddfil, mentioned in Captain Giraffe's answer, is the Swedish version of this.

Sour cream is the closest substitute, but it's much heavier, creamier and more sour than the real thing (40-70% fat, vs only 12% or so in kermaviili), whereas crème fraîche is also too fatty but not sour enough. Fatty unsweetened yoghurt or quark mixed with a bit of water will also do in a pinch.

If you can get your hands on some of the real thing, the recipe is simplicity itself: just mix with chopped dill, chill, and serve. Adding mayo would be unheard of in Finland, but a squirt of lemon or a dash of mustard would be common, particularly if this is to be served with fish.