In the bowl of a blender, mix the herring fillet and caviar, boiled peeled beetroot, cut into pieces, lemon juice, egg yolks, fried onions, mayonnaise. Whisk all this with a blender into a homogeneous mousse.
Capelin (pollock) caviar was added and mixed. Try to add salt. Add salt if necessary. Pepper to taste.
If herring with caviar, then the amount of capelin (pollock) caviar can be reduced or eliminated altogether. And you can put both.