Rengosalata is a traditional smoked herring spread. It's usually made with a combination of herring fillets (or herring roe), scallions, red wine vinegar, potatoes, olive oil, lemon juice, and dill. The herring fillets are puréed with the vinegar and scallions.
The potatoes are mashed and mixed with the herring purée and some lemon juice. The mixture is typically stirred with a wooden spoon and a bit of oil is drizzled into the mixture before it's covered and left in the fridge to chill for at least a few hours or overnight.