I had the same problem when I moved into this house. Seems the previous owner when he installed the microwave above the stove he positioned it level with the bottom of the cabinets so very close to the stove. They must have ate a lot of greasy food. I had my carpenters move the cabinet and microwave higher above the stove and that's when I found the oven light switch. Couldn't see it before. Anyway, I removed the metal filter and soaked it in boiling hot water with vinegar and Dawn dish soap (blue). It came out clean with a little help from my scrub brush/toothbrush I keep handy under the sink. Same went for the microwave innards as it was crusted with old food....YUCK!!!!!!