For the freshest and sweetest beets, look for bunches with the greens still attached. Once the greens and stems have been removed, your beets should weigh about 2 pounds. When shopping for herring, make sure to choose the kind preserved in oil, not vinegar, since vinegar-cured herring will be overpoweringly sharp in this salad. If you prefer your fish less salty you can look for packages labeled “lightly salted.” Two 250-gram jars or about one and a half 16-ounce jars will yield the 2 cups needed for this recipe.