Long story but I was in first class this June.  No one get excited it was a one-off.

Pretty much how ajas described. They'll keep offering more and more of it until no one wants a refill.  I have no idea the layout of the ovens on planes or how they use them in different passenger areas so I have no ideas on controls for contamination.  From what I saw (or did not see) nothing special no hand washing.  Just warming some nuts to feed the first class menagerie.

The nuts are typically WARMED in little ramekins (porcelain cups) in the "ovens" -- there is not a frying or open type pan and with resulting release of protein into the air, is my understanding.

Being over the ocean for long time with no place for "quick" emergency landing is always a consideration on these long haul flights . . . it's going to have to be individual decision as to reaction experiences and comfort//safety level and needs.

I am flying from USA to Australia.  I just found out and I am so anxious about these warmed nuts. I called United and the customer service rep. could not tell me how they were prepared (She told me to ask the flight attendant that day!).  I won't bring my daughter with a PN/TN allergy on the flight and could not find a nut-free flight anywhere.  So sad.

Air Canada will offer a buffer zone. Not sure how many hoops you have to jump through to get it though. (If a buffer zone is acceptable, you would have to transfer somewhere in Canada I assume. Probably no direct US to Australia flights, but I don't really know.)

Regarding liquids.

Liquids, gels and aerosols
The Transportation Security Administration (TSA) allows each traveler to carry on liquids, gels and aerosols in containers of 3.4 ounces (100 mL) or less, inside a single quart-size (liter-size) clear, sealable bag.​

My husband flies often, and he can buy drinks after going through security and bring it on. Expensive of course.  And that's domestic in Canada flights. Not positive if even that is allowed in the US.

I am flying from USA to Australia.  I just found out and I am so anxious about these warmed nuts. I called United and the customer service rep. could not tell me how they were prepared (She told me to ask the flight attendant that day!).  I won't bring my daughter with a PN/TN allergy on the flight and could not find a nut-free flight anywhere.  So sad.

YES, you CAN carry all the food (and drink!) you need through security -- but you would be wise to have physician's letter stating medical need and using the term "disability".  The TSA advisories address this at their website -- in the passengers with disabilites sections.  The "child under age 2" food rules won't apply, as the disability qualifies for special treatment.

And once you clear security, you make a bee line for kiosk that sells water/beverages to add to your carry on  beverage supply, if needed.  (Lots of people take empty water bottles through security and then fill them up at drinking fountains before boarding.)

You can also accept beverage in bottle/can from attendant and then wipe off, which might help some.  (Use own cups that you bring on board?)

The flight attendants always seem to be snacking in the galley area . . . especially if a "lull" in the action on board.  They typically are eating their own food that they bring on board.  With the pay-for-box stuff now on airlines, there is less "loose" food for the attendants to have access to (except those nuts as in the story above) . . .

I rarely see them wash their hands after they eat . . . and often they are snacking on food(s) as they restock or fiddle with stuff in food service area.

And in some aircraft the aft galley area is co-located with the restrooms . . . ick.

Well that settles that. Absolutely no anything for DS1, not even 'safe' prepackaged labeled snacks or drinks EVER, from any flight attendant. I'll get DH to buy water once he's past security for DS. Terrific inside info!