I am removing a double oven with a 240V breaker. I will be replacing it with just a single oven but would like to build in a Microwave below it. Can I put in a junction box and then pull a 120V line off on 1 leg and a neutral? The microwave would pull a lot less than the 2nd oven. Thanks

The single oven needs a dedicated circuit... so NO, you can't do it. And the microwave should be on it's own circuit as well.