This is a very interesting observation and I must admit that I have never looked into it before, so thank you very much for raising the question.

(1) In fact, at the beginning of the Sugya at the top of 54b, the Gemara asks "We have heard what the punishment is, but from where do we know that the Torah warns against it"? So the Gemara later on, which puts "Onesh" before "Hizheer", is only following the order of the Gemara at the beginning of the Sugya.

(2) However, it still requires explanation why indeed at the beginning of the Sugya the Gemara puts the punishment before the warning. I found a very interesting insight in the Sefer Hachinuch Mitzvah 69. He asks why Chazal always say "Onesh Shamanu Azhara Minayin?". Sefer Hachinuch answers that if the Torah would only have said that someone who does such and such a thing will receive such and such a punishment, one could have thought that anyone who is prepared to receive the punishment, is entitled to do so. This would be similar to some kind of business agreement - if one wants to do this one must pay a certain price, and then one may do it. Sefer Hachinuch explains clearly that in fact this is not the purpose of the Torah whatsoever. Rather Hash-m wants to keep us way from certain bad things and for some of these activities the Torah tells us immediately what the punishment is for doing them.

(3) Sefer Hachinuch also writes that this is what Chazal mean when they say (see Gemara below, near top of 56b) "Hash-m did not punish unless he warned in advance against doing this". Firstly Hash-m tells us that He does not want us to do a certain thing and only afterwards does He tell us the penalty for doing it. Hash-m will never give a punishment unless a bad thing was done. There is no such thing as getting punished for something which one may do if one wants.

(4) In short, by saying "Onesh" before "Hizheer", this stresses to us that if we receive a punishment, this can only be because we did something wrong.

(5) There is also a simpler answer that can be given to your question. All of the Mishnayot in this chapter of Sanhedrin tell us of transgressions for which the death penalty is received. Since the main purpose of the Mishnah is to teach about the capital penalty, the Gemara first shows the source for this in each case. It is only when we have explained the Mishnah properly that we turn to an issue that is not mentioned in the Mishnah:- what is the source of the prohibition?

Chodesh Tov

D. Bloom