I say either BASS-tyen or there will sometimes be a slight -ch sound in there, like BASS-chuhn. I’m not sure if that quite represents it but there you go. I actually do sometimes wish I preferred one or the other because it’s one of my favourite boys’ names but that ambiguity is a bit problematic (if I were to ever actually use it, and I nearly did for my son’s middle name). Anyway, wonderful name!