During the Burger Heresy, the greatest sous chef of all, Horus, burgermaster of the Mc'Imperium and second only to the Emperor, the greatest chef of all, was troubled.
The Emperor denied that the secret sauce existed. The Emperor insisted upon using the tried and tested methods of burger preparation. Only cold hard scientific fact was used in the cooking, and the cooking was fine indeed.
The Emperor personally trained 20 of his greatest cooks. The Frymarchs. They were the greatest and would lead a crusade across the world, setting up franchises in every city and bringing the McImperium cuisine to the masses.
But then the forces of CHAOS intervened, wary of the upstart McImperium and the new strange ways of the wok, they kidnapped the primarchs and cast them throughout the world. Each, afflicted with incredible amnesia, landed in a different part of the world. But each Frymarch quickly displayed their near mythical talent for the kitchen and soon became the dominant champion in the cuisine of their region.