For me, this is hands down the most authentic Finnish restaurant in Helsinki. This is where I go if I feel like eating reindeer stew or grilled whitefish. During the winter season (September to April more or less) they also have herring on the menu.

Cella is a true gem in Helsinki's restaurant scene. It's located out from downtown Helsinki, in the Kallio neighborhood. Cella is a classic, the prices are down to earth (meals between 14-21€) and they have a nice selection of beers. Cella has been open since 1969, and I think you should definitely pay a visit while you're in Helsinki.

https://www.ravintolacella.fi/