hmmm difficult.... in that case i would use bribery
this is what i would do:
give her a tiny packed lunch, that way there isn't too much for her to have to eat,
if it's all gone she gets a sticker. if she gets a sticker every day for a week she gets a prize
next week you make the lunch a tiny bit bigger, and do the samet hing
keep on until she is eating a good size lunch!