Lunch is in the middle of the day.
no, but i have eaten a big lunch two hours ago
Yes. They can be eaten together at any meal.
Since it is afternoon he has already eaten his lunch. (past tense). She will have eaten her lunch around noon time. (future tense).
Lunch or dinner.
Lunch and dinner
As a snack or a side with lunch and/or dinner
There are various things that Victorian school children might have eaten for lunch. For example, they might have eaten a block of cheese.