what is a crash-bang job?

To wallop is British slang for 'hit' or 'beat'. The piece is saying that the cook worked as if he was striking blows rapidly. 'Crash-bang' suggests a crash or collision (a car crash for example). His cooking style is rapid and dramatic.

Cen don't 'zak'l agree , tho you most always do better'n me. But I'd say 'crash-bang' here means just excellent, or outstanding

Could be, Dale. As impressive and surprising as a car crash, but in a good way. However you've got that 'wallop' at the start which kind of sets a wham-bam-thank-you-ma'am idea about how he cooks, so maybe a bit of both?

Update - the text is Australian relaxed/street/slangy style. It is from a novel called "Waiting" by Philip Salom. In Cockney and Australian slang you have the expression "crash-bang-wallop!" which vividly expresses a sudden event like a car crash, a blow, something falling heavily, etc. There are your three words used. Also, in criminal slang, a 'crash-bang' gang is one which specializes in robberies where sudden, unexpected and rapid action is the key. So maybe I stand by what I said. It's slang.

https://www.wheelercentre.com/projects/victorian-premier-s-literary-awards-2017/waiting

I must say it looks like a textbook example of bad writing, but what do I know?

Thank you Cen, you've made an old fella's day if not 'tire week

See updated. I think we both could be right.

Probly mor'n me

I had no idea about the language style and I never could have, because I'm not a native English speaker. So this can be very helpful for me in reading the rest.
you mentioned the name of novel, I'm wondering if you've read it and what is your idea about it? in total?

Where did you get the text from, that you quoted?

"waiting" novel

I have not read the whole novel, only the extract on the web page I gave a link to. I found it by entering the sentence you posted into Google.