My two-year-old is a fussy eater. How do I make sure that he is getting all the right nutrients?

My daughter is probably one of the fussiest eaters I've ever known. She was about 3 years old when the food tantrums started so it was difficult for me to "hide" the good stuff into her food because she could spot them. What I did was to firmly tell her this is would be her meal and there would be no other if she doesn't eat up. If she misses her meal or doesn't finish up, she gets nothing except snacks (once in between meals). The only snacks we have in the house is usually blueberries, celery or fruits that my mum would buy. I admit, this sounds harsh and is quite difficult to put across to a 3 year old but after a week, it seems to get easier and easier.