Seven said:
Boil in a pot first and pour it into a sanitized flask later if you like. Or boil in the flask. Either method will work.
As pointed out already, spending the cash for an E-Flask if you don't intend to boil in it is silly. You can get a much cheaper starter vessel that doesn't need to be able to withstand a direct flame.
And that's really why E-flasks are the only way I will do it. As effective as Star San is, it still doesn't hold a candle to the effectiveness and utter foolproof-ness of 212°+ of wet heat. Boil the starter wort (water + DME), yeast nutrient, some fermcap, and even the stir bar all together - just make sure to add the stir bar before you start boiling, as that Teflon causes it to become an instant volcano. If you can fit the flask into a pressure cooker and "autoclave" the entire thing, even better... even boiling will leave about 0.00001% of microorganisms alive, which may not seem like much, but when you consider the purpose of starters, they don't really discriminate between the desired yeast strain and everything else.
Heck, I even leave my foam stopper in for the latter part of the boil to both sanitize the stopper AND to create a closed environment to ensure it remains sanitary while it cools. I've seen people here dismiss the foam stopper, despite even the actual experts (eg Chris White) touting it as the best solution for keeping the flask sanitary (as opposed to foil and ESPECIALLY airlocks), since they claim that the foam stopper is still too restrictive to allow gas exchange with the CO2 generation creating positive pressure, but nothing's convinced me of just how wrong those people are more than boiling wort with the stopper plugging the flask, and watching steam pass RIGHT through, as if there's nothing even there, with none of the "building up" you'd expect if gas exchange were in any way impeded.
I have two E-flasks - a 2L and a 5L - and I got them both from MoreBeer. So while they're decent, they're hardly the best of the best. And yet I'm constantly boiling with them without any problems. The main thing, as I mentioned, is to *start as low as possible* and gradually increase the flame, in order to reduce the thermal shock. I do this more for safety reasons than economical ones though - since I'm boiling on a stovetop, I'm more concerned with liters of boiling or near-boiling wort spilling out than having to buy a new flask, but in a way it's actually even *more* important if you brew outdoors and spilled wort isn't much of a concern, because outdoor burners typically put out *way* more heat (often more than an order of magnitude), and so the chances of breaking a flask are much higher.
And since safety is really my main concern, I don't really have a problem being a bit more carefree while cooling - the flask comes off the flame, and directly into a 32°F icebath in my sink. If it somehow cracks and spills, it's both contained and immediately cooled down. However, ALL the stories I've heard here of cracked flasks have been while heating - not a SINGLE ONE while chilling - so I strongly suspect that they are more capable of withstanding contraction than expansion.
