I think one can also use "better" as a verb here, instead of 'improve'. "She betters the lunch",
I would never use 'better' in this way in Midwest US.
It's quite an English thing
Hmm - not really. For me (English), better as a verb is to do better than (someone else). You better your competitor, not the thing you're improving.
"She makes lunch better" doesn't suffice?
It seems to me that "she makes THE lunch better" could be a correct translation. I would typically stick to improves though because from what I've seen, better (melhor) is used differently.