Way OT. Reduced to carbon

I've had success with baking soda (bicarbonate), soak overnight with a thick paste applied to the burnt patches.

Try "oven cleaner".

else put a twist-knot wire brush on your angle grinder. (10 minutes elbow grease)

Pure oxygen or O3 at a goodly temperature is probably a number one; just be careful about removing possible CO. Maybe burnt water (hydrogen peroxide) as a possible number two. Bleach is a number sixteen with damn little between; incomplete conversion and requires that animal fat you mentioned..(and may still be incomplete).

Yeah, hot Na(CO3)2 is a slower reagent than lye (NaOH), and much preferable if there's any aluminum involved. My trusty popcorn kettle has survived for decades with bicarbonate hot baths after each wildfire. It cleans ovens, too, just might take several applications.

I hate to use it on frypans, though, because it removes the 'seasoning' (even from cast iron).

You pop corn over wildfires? Bold man!

"Sir, we are evacuating this area, you have to leave NOW!"

"I can't leave, my popcorn isn't done yet!!!"

"Oh, well that's ok. We'll be back to find your ashes then."

That kind of thing comes off easily if it's hot. Just heat the pan and then scrub with dry steel wool and a pot holder.

Lye or a lye based degreaser (industrial Purple for example) will loosen it if it's not too well baked. Barkeepers Friend, thick paste, rubbed, will wear through it after a fashion. 600 grit wet-or-dry sandpaper with a light oil like WD-40 will get it off. After that a polish like Wenol