Which is correct:
1-I mentioned that when we were eating lunch.
2-I mentioned that when we ate lunch. (Here meaning 'while we were eating lunch and NOT after we ate lunch)
3-Mention that when you are eating lunch with him.
4-Mention that when you eat lunch with him.
All four are possible. Although while and the progressive tenses are usually associated with longer actions, I don’t think the distinction is observed very carefully by native speakers.
Mmmh… Our dear MM is very mysterious indeed – always so absolutely and strictly professional… and so secretive! I guess it all adds to the suspense, though!