Process manuals and recipes kept internally, engineering change logs, quality-control procedures, any patent or trade-secret policy, and training records showing the method is taught inside the company.
Let the paper trail show the knowledge lives inside the company
The strongest evidence is documentary and pre-dates the petition. Internal standard operating procedures that differ from the equipment maker's manual, dated change requests showing how the line was modified and why, batch records demonstrating the company's own parameters, and a confidentiality or trade-secret policy that treats the method as protected all show that the knowledge is the company's. Training rosters and onboarding plans show how long it takes a new hire to learn the method, which supports the claim that the knowledge is not readily transferable.
For the technician himself, maintenance logs, sign-offs on process changes and records of him training others place him inside that knowledge. Where documents are in French, add certified English translations. Do not create new documents to describe the process after the fact and present them as historical; write a current, honest technical statement and let the dated records support it.