Soda crystals - I note that Azula R has suggested baking soda, don't think that's quite the same thing, because household soda crystals aren't meant for cooking use. Switch the power off, make up a very strong, hot solution and pour into the hob so that it's as full as it can be without overflowing. Leave to soak for as long as possible,couple of hours at least, top up if it evaporates. It should mean (provided you can get the solution depth high enough) the solidified oil/grease just lifts off. Bit of a pain to remove because you can't lift the hob to pour it, but it can be done by sopping it up.