식사 can't really translate to "dish" I don't think. A dish can be a specific meal, like when you say "There's a new dish on the menu." but it can't be used generally like 식사를 먹는 중이야 - I'm having a dish right now. Sounds weird I think. You could say 요리, however, as "dish".
That reminds me, 메뉴 could also be accepted as "dish", because they use it differently (and/or wrong) in Korean to mean a new, single dish on the menu as well as the menu booklet.