Theres the old trick of putting bits of the oven out onto the grass during the night, the morning dew suppose to break down the great, either that or the wildlife have a party on all that grease, it used to work well on Bar B Que stuff, otherwise, just Soda Crystals and washing powder + strong arms and a scourer...Soda is horrid stuff on your hands so rubber gloves...must clean my oven, my man cleans up well after cooking but doesn't cook very often!!!