How do I recover bch sent to external btc address? I mistakenly sent bitcoin cash from my bch wallet to an online casino but I sent it to their bitcoin wallet instead of their bch wallet and they didn't receive the funds to credit my account. Can someone please help me recover my bch if possible.
Most likely, you can't.

I am going to assume this happened recently.  If you recently sent BCH to a BTC address, then most likely it was sent to a segwit enabled address. If that is the case, they are probably gone and locked up forever.

IF you sent them to a legacy address, they will show up in the wallet. BUT, there may be more problems.  If the client that contains this wallet is segwit enabled, you CAN'T and should not attempt to just send them back without first disabling segwit.  

The best scenario, is they were sent to a legacy wallet address on a legacy client. i,e. no salt, no segwit, etc.  Then you may recover them.  

No matter what the case, it is worth at least trying.  You can try with one of these. https://bch.btc.com/docs/help/bch_segwit_recovery

Good luck!