 First of all, you want to soak the grids in hot, soapy water, because soaking them will definitely break up all of that baked-on, caked-on yuck, and then clean them with a nylon scrubbing pad. If the grills are really encrusted, you can use an oven cleaner in a well-ventilated area and then rinse clean. Next, you want to remove the lava rock or the ceramic briquettes and clean them with a wire brush and replace any that are deteriorated.