i assume you mean the inside of the lid.
here's what I do.
1. spray inside with easy off oven cleaner, make sure you coat it really good.
2. put the whole lid inside a black plastic trash bag.
3. leave lid, inside trash bag, outside in the sun for the whole day.
4. after a bunch of hours in the sun, open bag, hose off inside of the lid.
5. use a plastic putty knife to scrape the thick stuff.
if needed, repeat.
if you have an old hard drive laying around, cut it open and remove the hard drive platters. they're highly polished and very smooth. they make PERFECT scrapers for the inside bowl and lid.