I've had great results with the excellent
Shure MV88+ Video Kit hardware + software, if you don't mind attaching extra hardware to your iPhone.
It's got a very high-quality mid-side capsule that lets you tweak stereo width while also mixing to mono well. Comes with a wind screen and supports variable-HPF for outdoor use. Surprisingly high quality kit with neoprene travel case, aluminum tripod, sturdy-ish iPhone clamp that lets you orient it (and the mid) for portrait or landscape video, and cables for iPhone lightening, Android microUSB, or USB3.
You can configure the mic using Shure's recording app, then use any other iPhone app for recording/streaming with it, or use Shure's audio or video recording app.
The only downside is that you can't power the iPhone when the mic is plugged in, so your recording length is limited to your iPhone battery.
(Note that the MV88+ is a totally different product than the MV88)
FYI, the MV88+ looks like it's $50 off this weekend, and is a great deal at $200!