I must say that in my experience both in my past life in Greece and my much wider experience over the last few decades speaking to Greek-Americans or Greek expatriates the preferred translation for this not what are you doing but rather what are you making or what are you fixing or preparing or constructing depending on what the specific situation is.
But to hear this in isolation I would never say what are you doing as a translation for this. I could understand the situation where it would arise. To me it seems like a slang expression. Here again of course I am far removed from everyday usage of Greek has spoken in Greece.
My comments are only from my perspective.
I think "making" or "producing" should be the right translation for "φτιάχνω".. I am taking Greek lessons from a native Greek professor since 1.5 years. We are using φτιάχνω also for cooking, for actually preparing the dish in the kitchen. We have never used this verb to say "what are you doing?" Maybe the correct translation for this sentence should be "what are you making ?"
The form of ποιέω is not used in modern greek. Ποιώ is there but sounds archaic when used on its own and its usage is mostly about "creation" and not about "preparing meals". Ποιώ is used in everyday life as a second component of compound words, for example Χρησιμοποιώ =use, μεγαλοποιώ = magnify, exaggerate etc. So you can't really say Ποιώ for preparing food. Φτιάχνω is the verb used on its place in modern greek.
I don't see "τι φτιάνεται" on our list of hints for words to be used to translate this sentence. But it doesn't fit here because it is "Passive Voice" meaning "what is made"/what is being made".
What we have here is simple present (present indicative) just plain "what do you do /make/fix"