First off, I apologize in advance if this is a repost, search yielded nothing. I also want to apologize as this guide is somewhat situational, and you may not apply.
First off, this requires you to either be on a Normal server, or have a second account (or possibly a friend, etc). The easiest and most popular way is most likely going to be via making a Deathknight, so it may also be required to not have a Deathknight previously on X realm.
Now, onto the actual guide. The main bread of butter of this guide, is a cooking pattern off an alliance only quest, Recipe: Tasty Lion Steak
While the quest is alliance only, the recipe itself is not BoP, it can also not be purchased from any vendor (even for alliance). Meaning you can only get one per character. Said quest, is Costly Menace. A low level quest in Hillsbrad Foothills.
Now, of course this all depends on each individual server, but on the 3 that I regularly play on, this recipe sells for 1-2k (It's extremely varied, lowest I've seen one go for is 600g, though; 2k is the average).
The easiest way to farm this, is create a Deathknight, do all the prequests, then hop on over and complete the quest, transfer the recipe via neutral AH; rinse and repeat. Now, I haven't tried not doing all the prequests in the Deathknight starting area, but I'm about 90% sure you won't be able to accept the quest otherwise as the person who hands out the quest is associated with Stormwind. Either way, to do all the prequests, as well as the actual recipe quest, takes around 3 hours. However, for those that are slower, I figured it as 4 hours (Which is more than enough, if you can't do it in 4, you're doing something wrong). Hence where I got the 500g/Hour (remember, 2k on average).
Also, on one final note, for those of you curious just why this goes so much, is purely for this achivement Chef de Cuisine. There's barely over 160 recipes in the game, almost requiring this recipe for most classes.