It’s a dish from (Soviet) Jewish cuisine. My version of Forshmak doesn’t have the butter. In the classic recipe you need to add 1 stick of unsalted butter for creamy texture. This dish go well on Russian rye bread with sweet hot tea.

2 herring fillets, low-salted, not marinated
4 hard-boiled egg
1 sour apple (Granny Smith is fine)
1 medium onion (sweet is ok, but you need half of big bulb)
2 peaces white bread, soaked in milk  (I used gluten-free bread and rice milk)
1 tbsp olive oil
1 tsp apple vinegar
1/2 tsp sugar

Peel and cut the apple, onion, and eggs
Blend it all together, including the soaked bread very well. Cool in the refrigerator.