With marble I usually use white modified thinset and have no problems. Yes, even on Kerdi.
Depending upon the particular onyx you may be able to use the same thing, just make sure you get full coverage so any imperfections in the thinset do not show through since most onyx is translucent.
Each brand of onyx has specific recommendations as to what to use to set it. If it says to use epoxy I would check with Schluter to make sure that won't create problems - I've never done it so I can't say.
Depending upon the particular onyx you may be able to use the same thing, just make sure you get full coverage so any imperfections in the thinset do not show through since most onyx is translucent.
Each brand of onyx has specific recommendations as to what to use to set it. If it says to use epoxy I would check with Schluter to make sure that won't create problems - I've never done it so I can't say.