The earliest known document mentioning the potato omelet was a 1817 letter to Navarre describing residents of Pamplona consuming eggs and potatoes mixed into a tortilla. Some credit General Zumalacárregui with inventing it as a simple, nutritious dish for his Carlist army, while others attribute the invention to chef Lancelot de Casteau, who published the recipe in 1604 while cooking for bishops in Liège.

2. Until recently the first known document in which reference was made to the
potato omelet was a letter dated in 1817 addressed to the Cortes of Navarre
in which it was spoken that the inhabitants of Pamplona consumed: two or
three eggs in tortilla for five or six mixing potatoes, bread, etc. Some
suggest that it was General Tomás de Zumalacárregui who invented the
tortilla de patatas "by necessity", since it was a simple and nutritious dish
capable of satisfying the appetite of the Carlist army. Others propose that
the invention should be attributed to the Belgian chef Lancelot de Casteau,
who published the recipe in 1604 and who worked as a cook for three
bishops of Liège.