Lerping cannon rotation

Hello,
I have a simple question. How to replace this line of code, made in non-deterministic behaviour with code made in a True Sync, deterministic way?
transform.rotation = Quaternion.Lerp (fromRotation, toRotation, turningSpeed * Time.fixedDeltaTime);
I have tried lot of ways but it didn't work, so if you know the answer please help .