Sounds like it should actually be "χonch," then, with a Greek letter chi instead of an 'x'.
xonsh being pronounced /kɑnʃ/ and not /eks.ɑnʃ/ or /xɑnʃ/, or even /zɑnʃ/, is just one of those things I have to remember.
pronouncing TeX /tɛk/ is better than /tɛks/, but really it's /tɛx/, the way Bach is /bax/.