My advice (I have lots of experience in this area ) is to not push it.
My dd lived almost entirely on milk, raisins, dry cornflakes and unbuttered bread for a while - dry cornflakes and the bread were my alternatives to the meal I was offering.
She didn't even eat chips (I was confused, I thought all children liked chips) until she was 4, roast potatoes until 5 and yorkshire puds until 6 - all things she now loves for an occasional treat.
I didn't make that big a deal about it, she got over it slowly and now eats almost anything, has very healthy attitude to food, always eats her 5 a day and will try all new foods (she is 8).Sprouts have been her fave veg since she was 4.5.
My DS (3.5) has a very limited diet of about 5 main meals - no veg at all although eats bananas and raisins - and 2 breakfast cereals - he will eat sandwiches etc - life can be very boring. He is quite small and skinny, but has SN inc health problems including a glucose metabolisation problem. I would rather he had jacket spud and cheese for lunch every day than ate nothing.
Ask your Paed if you can be referred to a nutritionist, who will probably reassure you that she is getting what she needs.
Think about what she eats and give her the best you can. DS liked fish fingers, so now I make organic salmon fillets sliced and covered in crushed cornflakes and he will eat several - perhaps half a small fillet?
DD used to like chicken nuggets (I blame the nursery) so I bought M&S organic chicken breast goujons or made them - so that instead of reclaimed meat she was getting real meat.
They both like pizza so I make my own, including the tomato sauce. I put grated carrots and finely chopped celery in the sauce and the mouli after it has been cooked to make it smooth, they can't tell the difference.
If your DD likes spag bol or cottage pie, make it with really good lamb mince (British of course) - it is far fattier than beef so extra cals.
With mashed pots/jacket pots add loads of cream, butter and cheese every calorie counts.
Also I let my DS 'help' me whenever I can with the cooking, he will stick alsorts in his mouth and try it - he ate a matchbox sized lump of stilto the other day!
Hope these ideas help.