You don't say what your use case is - personally I think that 15 minutes is a very long time before you detect that the processor has got stuck.
Be that as it may, you could set up a timer within the ESP32 that you reset on as regular basis by testing whatever it is that you are concerned about getting stuck. If that times out then you can do a software reset.
If you are concerned about the ESP32 itself getting stuck, the you can also use one of the internal watchdog timers. While they typically go up to 60+ seconds.
By the sound of it you are wanting an external circuit. You then need to tell us about the other design parameters that you can deal with - for example budget, PCB space, power etc.. Most important is what is the external thing that you are monitoring and how does it tell the external watchdog that it is still active.
Finally, what have YOU come up with? It is much easier for us to assist you if you have thought the problem through and com e to us with specific quesitons to actua problems.
Susan