i soak mine in vinegar and baking soda and it typically takes a little elbow grease with a scoring pad or brush. the vinegar and baking soda will bubble and loosen up the burnt on particles. you can also heat it up a bit on your stove top.
my go to is to make a paste of vinegar and cream of tartar. the dry stuff, not what you put on fish:) wipe this on the gunk, let it sit for about a minute and then scrape it off. depending on the thickness of the gunk, you may have to do it a couple of times. rinse well and that's it. hardly 