I want to owner finance my house to someone and I have a mortgage on it now. Can I do that simply and legally.

Welcome to AMHD. Review your promissory note and mortgage; look for what is called, "a due on sale clause". Check this: Due-on-Sale-Clause

You can not directly sell it, because you would have to pay off your mortgage, you can do it under a "contract for deed" or a lease to own.