What is the proper way to thread tornado+flask app?

Tornado's WSGIContainer does not support threads, which means that it is also not suitable for most production usage (the rest of Tornado is, just not WSGIContainer). It's only a good idea to use Tornado's WSGIContainer when it's important to serve both Tornado RequestHandlers and WSGI applications in the same process. Otherwise, I recommend gunicorn or uwsgi.