I agree that this a "behavior" thing, but I wouldn't call it a problem. It doesn't sound like the nutritionist has worked with many toddlers. Toddlers graze as they go, and that is about the best you can hope for some days. Every single thing I have read about toddlers' eating habits at this age talks about this exact thing. It's a control thing, toddlers have very little control over anything in thier lives, and they learn pretty quickly that they can assert control over what they put in their mouths. 15 mos is exactly the age my son (who was a fabulous eater before) started doing this. And it changes constantly. One day he loves waffles, next day he won't touch them. So seriously, don't worry too much. I tried never to make mealtime a battle- not worth it.
Mine is much better now (at 19 mos old). One thing that helped me get through this was allowing him as much control as I could over other things. Mine has been a pretty early talker, so that helped. But I let him pick between two choices (for food, toys, books, everything), help with things around the house, and any time I can see an opportunity to let him "decide" something. That really helped.
A couple of healthy food suggestions: flavored yogurt has tons of sugar. I make my son plain whole milk yogurt every morning with some fresh or thawed frozen fruit in it. If you need to make it sweeter than the fruit is naturally, adding a little sugar to the plain kind is still less sugar than the store bought stuff. If you try to stick with the healthier versions of crackers and cereal, these arent that bad. I do plain graham crackers and cheerios or Kix. I would avoid the fries, or do homemade oven fries. I never let my son even try stuff that I don't want him to get hooked on. My sister says I'm mean, but I figure he doesn't even know what he is missing. And then I don't feel bad when I let him have pie or ice cream on special occasions. And the natural sugars that fruit or all-fruit juices with no sugar added are fine. There is nothing wrong with eating fruit sugars, the refined stuff is what is bad. My ped said to let him eat as much fresh fruit as he wanted as long as it didnt upset him stomach. I would talk to your pediatrician and see what they think, but don't stress too much. It'll pass if you don't make a big thing out of it. Hope this helps.