In a Diffie Hellman setup, configure the machine you aren't sat in front of (usually the server) to use a fixed secret value instead of a random one.
Now since you know this value, and the other value you need (from the client in this case) is sent over the wire, you can run the DH algorithm and decrypt everything.
You should (obviously) never do this in production, although it is what various financial institutions plan to do and they have standardised at ETSI as an "improvement" on TLS (you know, like how TSA locks are an "improvement" over actually locking your luggage so random airport staff can't steal stuff) ...
Now since you know this value, and the other value you need (from the client in this case) is sent over the wire, you can run the DH algorithm and decrypt everything.
You should (obviously) never do this in production, although it is what various financial institutions plan to do and they have standardised at ETSI as an "improvement" on TLS (you know, like how TSA locks are an "improvement" over actually locking your luggage so random airport staff can't steal stuff) ...