How old is she? My son used to be that way! He'd only eat certain things but once he got to be a teen he started trying all kind of new stuff! He's 18 now & even eats broccoli! Yeah! The one bite is good! That's how we got our son to start trying things! best to do it while they're young!
I'll advise you to take her to the nutritionists, for her to be told that she needs new types of foods. This is because her health might deteriorate because of the one type of food she is taking. Give her more fruits, so that they can compensate for the lack of proper balanced diet.
We use, "One bite, it's the rule" at our house. You have to try just one bite, but, if you don't like it, you don't have to eat any more. It gets our toddler to try everything; sometimes he'll like it and eat it, and sometimes he'll spit it back out, but we both have to respect that it's just a "one bite" rule.
This is an interesting question because I asked a question that is similar to this one in nature asking if it was OK to "starve" a child into getting him or her to eat certain foods a long time ago. (The question is NOT as barbaric or draconian as it appears!) I originally asked the question because I used to be a picky eater myself somewhat, and then I discovered that genuine hunger can perform miracles in changing finicky eater's minds. So, that is my answer if it means that much to you that the kid eat something else besides ??--Starve em.
Food is not a great tool for lessons or bribery. They should try a bite and they eat or not. try it again in a month taste is developed and tastes change. If it becomes a contest then they may never try it again in future.