Pour the frozen pelmeni in 8 1/3 cups (2 L) boiling water. Add the broth and 1/2 the butter. Wait for the water to come to a boil again, take the pelmeni out after 5-10 min. and leave them to cool.
In the meantime, get to work on making the cream sauce. In a pot, melt the other 1/2 of the butter. Add the sour cream, chopped onions and spices. Stir and once you obtain a smooth sauce, remove from the heat.
At this point, you can proceed in 1 of 2 ways. One option is to add the pelmenis to the pot with the sauce, stir and then serve.
The other option is to distribute the pelmenis into plates and pour some of the cream sauce over each serving.