Agree with pps but if she is still hungry and has left as you say a larger portion of sandwich along with the crust I would show her and identify the soft part and the crust part and suggest she has another bite if she is still hungry. I would also tell her it's ok to eat the crust. I see mine go through phases of actually being a little unsure what is to be eaten and what is to be rejected, so for instance leaving an apple core or a corn cob. Even these days he might ask me about certain things, "what's this?" as he holds up skin from a cooked tomato (he loves toms) and loos slightly disgusted that I've served him something sub standard, I said "it's tomato skin, it's ok to eat it" and in his mouth it goes. I never make him eat something he honestly doesn't want to try or that I know he doesn't like but I feel sometimes they need information too.