It's pretty normal for them not to eat their lunch properly when they first go to nursery- eventually they learn. Your best bet might be to try a bento approach- little bits of various things to tempt her into eating.
DD's might have some combo of raspberries, sliced grapes, pretzels, sandwich, crackers and cheese, salami slices, a yogurt, a mini soreen loaf, paparami, felafel, wraps, (cold) mini pizza, pasta salad, an apple, blueberries, a bagel with filling, mini cookies, veg and cheese muffin, pita break, hummus, red pepper and Philadelphia dip, carrot sticks, cucumber sticks, cherry tomatoes (sliced), sliced olives, rice salad, cream cheese and ham/salami pinwheels, dry cereal (her choice!), cold chicken/roast meat leftovers, cheese sticks...whatever is in the fridge/cupboard and suitable, really (we normally include a complex carb, a dairy protein, a non dairy protein, and at least one portion of both fruit and veg just to ensure she gets what she needs)