This dish was one of my favorite when I was a teen. I remember making it pretty often during my first years in the U.S. when all of us were homesick. We lived in Sunnyvale, CA without a car for more than a year with the closest grocery store was Asian on Duane Ave, so getting good whole mackerel was not a problem. I forgot about this dish for years until now.
Not sure how widespread it was in the Soviet era and what variations existed out there. We discussed it in LCL Group on Facebook and, apparently, the recipe with tomatoes was more popular. In other regions, pink salmon (aka Gorbusha) was more available than mackerel and was cooked similarly. The recipe below is how my Mom made it. I loved eating creamed mackerel with vegetables as a cold appetizer after school. My favorite part of this dish was the vegetables — naturally sweet, slightly flavored with sea salt and umami, and rounded with silky cream. They had to be soft and barely crunchy.
Everybody knows everything tastes better when cooked with bones. Unfortunately, many people are afraid of or feel uncomfortable eating food with bones for many reasons. They have an option to use mackerel fillet. If mackerel is way too flavorful for you or not easily available, substitute it with a salmon fillet.