First of all, you should never let it come this far.
To prevent cast-iron cooking utensils from rusting, always clean it after using and then rub in a thin film of fat/oil. This way, the items can last a hundred years.
I was very lazy and then forgot about my pan.
(btw this is the lid of my dutch oven, but it works great for frying eggs and things)
When I wanted to make tacos, it looked like this:
Probably not a health risk, but disgusting.
In Australia, they have massive fixed hotplates on campgrounds, that never get cleaned.
This is where I developed this method with a friend.
First, you need a fire. Bigger than you would need for cooking:
Once the fire is blazing, heat up the pan and add cooking-oil and thin combustable stuff like dry leaves or hay (whatever you can find). If it is hot enough, the oil will catch fire and the organic material will wick the oil.
Done correctly, everything in the pan should burn down to ashes.
If everything worked well, you can just blow the ashes away and you have a clean pan.
If not, repeat; Crank the heat more.
Wipe it out some and it is clean. Also absolutely sterile.
Ideally, you can then fry potato slices to properly burn in your pan.
I gave up on that - I find it wasteful and pointless.
At first the pan might stick a bit, but that is ok.
If you are roasting tomatos like I did, it will eat through the patina anyways.
It is not a problem. I fried meat in this pan later.
If anything sticks to the pan, just wait. once it is fried crusty, it will come off.
trust me: it will come off before it's burned
This does not work like your teflon no-stick pan at home.
It took me very long to accept this, but even if you think it's burning - if it is still stuck to the pan, it is just not ready yet.
Now you can prepare food outdoors using old sticks and things.
If you use sticks that just lay around, they would have decomposed to CO2 anyways; whether you burn it or let it rot, the carbon footprint should be exactly the same.
This is how it's done.