What else qualifies (steel, 15 round mag) other than a Bren Ten or Tanfoglio Witness? So for all intents and purposes, you have one choice: a Witness. I have a 10mm "WonderFinish" Witness -- no complaints, it's a right nice gun. The WonderFinish looks like stainless, but it's not actually stainless steel. 15 round magazines are easy to get now that the ban has expired. 10 round magazines can be easily modified to 15 rounds, although the modified mags limit your OAL a bit.
If you like 44 mag revolvers, there's always the S&W 610. It's full sized, it's stainless, but it ain't 15 rounds.