Roll the paste out on a floured board and divide in two rectangular sheets of equal size. On one sheet of pastry put a layer of the cooked kascha or rice, mixed with the chopped onion and chopped mushrooms, also previously cooked. Then lay the slices of cooked salmon on this, and cover with another layer of kascha. Over this put the other sheet of paste, damping the edges slightly and pressing firmly. Make a slight opening in the centre of the koulibiak, so that the steam can escape, brush over with melted butter, and put in a moderate oven for about 45 minutes, till the paste is of a golden colour.