Vaughan Williams wrote a handful of operas. Shostakovich wrote two.
Schubert wrote a whole bunch, but they've never made it into the repertoire.
Weber wrote a couple of symphonies. Nielsen wrote at least two operas.
Haydn wrote some operas. Mozart, of course. I'm sure there are others.
By the way, if you want to get your feet wet in Prokofiev operas, I'd
suggest that the Fiery Angel is his masterpiece in the genre. If you
look hard enough you can even find a good video production of it on a
now discontinued Philips VHS.
Richard, who invites you to visit him at http://opuspocus.ca