"I eat one fish sandwich."
Translation:Ich esse ein Fischsandwich.
No, no, it doesn't change depending on what's on the sandwich. ;o) The problem is that "Sandwich" can apparently be both, masculine and neuter. Although I have to say, I find "der Sandwich" very awkward and have always heard and used "das Sandwich", in my opinion you should stick to that.
The indefinite article ein is for both masculine and neuter nouns, by the way (feminine: eine).