Have a 'picnic' lunch at home on Saturdays. If you can't get outside because of the weather, have it on the living room floor. Or kitchen floor even! Make it fun and interesting, invite some teddies. Put out little bits of different things and find out what she likes.
A little bit of bread and butter. Tiny strips of grated cheese, little teaspoons of peanut butter, crackers, rice cakes, mini rolls, wraps, pasta, rice, sticks of carrot, cucumber, celery, peppers, grapes, raisins, dried fruit, little yoghurt pots, etc. Not all at once of course, try a few at first and see how you go.
Once she enjoys the social aspect of the picnic and you discover what she likes and will eat, she may enjoy trying new things which you can add to the list of lovely foods. If she doesn't like or want some of the choices, cross them off the list for now, but reintroduce after a while because tastes do change.
Don't overfill the lunch box, it's best that she has a manageable amount to start with. They usually get about 20 minutes to eat I think.