This is typical Swedish midsummer food!
Herring is a nice and fatty fish, perfect for keto dishes. The pickled ones can have too much sugar, but as long as you don't add the pickle liquid to your plate, you'll be fine.
My summer dinner today consists of
onion pickled herring
2 hardboiled eggs, still warm
one tablespoon butter (added on top of the eggs to melt)
a dollop of créme fraiche
some green leaves from a friend's garden
and chives from my balcony
Simple and very, very good!