A friend just takes photographs of everything they eat, and goes through the google photostream later to log things in a Bullet Journal (which I think is a wanky time-sink, but seems to be less of a distraction than an electronic equivalent might be), along with symptoms, sleep quality, etc.
This seems more reliable than getting the journal out and logging food at the time, particularly when eating in a hurry to manage medication side-effects. Your ADHD may vary.