A Matryoska Gestalt was a being created from several people being merged together. They were logical absurdities and technical impossibilities which interested both the Great Houses and the Enemy.
When Jamie was trapped in 1990 Stevenage, he used a reality-cutting knife to amalgamate Alison, Michael, and Laura into a Matryoska Gestalt. Jamie planned to sacrifice the Gestalt to tear a hole in space-time and make a portal to the Eleven-Day Empire, but he was overpowered and killed by the Gestalt. The Gestalt then tried to continue living an ordinary life in Stevenage. (PROSE: Office Politics)