When: I mentioned that when we were eating lunch

[Navi]

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.

[Mister Micawber]

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Hi Navi-- welcome to English-test.Net

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.
_________________
Native English teacher at Mister Micawber's

[Navi]

Thanks a lot Mr. Micawber!

It is a small world. I know you answered questions here too. I had seen your name, but I didn't expect to 'run into' you so soon!

[Mister Micawber]

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Just call me 'Speedy Gonzales', Navi.

See you!

MM
_________________
Native English teacher at Mister Micawber's

[Navi]

I wonder if there is only one of you!

[Conchita]

[Tom]

[Conchita]