... I am going to have to go through all 350 pages and mark every time Apple appears in order to get it into my index properly.
That is how a professional indexer would make the preliminary list. (And after generating this preliminary list, this hypothetical indexer goes over the list again and again, concatenating, merging, deleting, and moving items around. Otherwise it would be a simple word lookup list.)
But aside from that:
Is there a simple way to have InDesign find all the occurences of "Apple" (and "Bean" and Banana", etc.) without my having to individually mark each and every occurrence?
There is a button "Add All" in that very same dialog you are using to "Add" a single entry. Guess what it does.
JW: cant you write a script to auto index ...
I agree: leave indexing to a pro... i (90%) of the time hand it off... a cookbook? then the 350 pages wont have too many words... so wont be expensive... 3 days for the indexer... maybe.
A text concordance works very well for the Simple Cases (i.e., in this example only ingredients). That's what I supply to my clients when they ask for "an index", and with it, the advise to go over it carefully.
Marc Autret has some scripts that deal with various kinds of indexes. One example is http://www.indiscripts.com/post/2011/07/indexmatic-2-public-release-and-user-s-guide but I couldn't say if this is something the OP could use instead of ID's native indexing.
A text concordance works very well for the Simple Cases
hmmn... i suppose the client can pick out what he wants indexed..
I usually (when its a low budget project) ask the client to send me an exel s sheet... with his idea of how/what he wants indexed... (ugh i know) .. indexing time consuming no matter what approach is my point...