Here’s how I cleaned this grill pan.
The simplest way to clean the pan is with a lye-based oven cleaner, like Easy Off. The dollar store has their own version, which works the same way. Easy Off is a foaming detergent with lye.
The lye mixes with grease, to form a soap. The soap is water soluble, and rinses off.
Basic Instructions for Cleaning Cast Iron with Lye
Get a large plastic bag, like a kitchen trash bag.
Put the pan into the bag, so you can tie it closed. Don’t close it yet.
Take a deep breath. You don’t want to be breathing when you spray the oven cleaner.
Spray the inside of the pan with the oven cleaner. Get a good 1/4 inch thick foam in there. Do not breathe in the chemical, because it’ll burn your lungs. You should also wear gloves, at least until you get the hang of this.
Let it sit a few minutes so the vapors are released and the foam relaxes. Then, put the whole thing into the plastic bag, and tie off the end.
Go wash your hands! Let the area air out, before you breathe in too deeply.
Put the bag and pan into a well ventilated room, like the bathroom. You can also leave it outside, if it’s safe. Wait overnight, and then remove the pan. The oven cleaner should have turned into a brown liquid. Rinse that out.
Take the pan inside, into the kitchen, and scrub it with coarse steel wool. This should scrape off even more of the black gunk.
If there are bits of carbon left on the pan, repeat the spraying and cleaning.
The second time around, I sometimes clean off the bottom of the pan as well.
After the second cleaning, I generally don’t bother to try removing more carbon. When it’s stuck on like that, it means there isn’t any soft grease underneath the carbon that the lye can react with.
The only way to remove that carbon is to throw hot charcoal into the pan, and let the charcoal ignite the carbon bits, turning them into ash.
To reseason the pan, rub it with a little oil, and heat it up until the oil starts to smoke. Remove from the heat to let it “dry”. It’s actually just polymerizing and hardening. You can repeat this a couple times, but once is enough to protect the pan from rust.