I have seen this question posted on here before but the answers are always always water, vinegar, baking soda, or lemon juice. I have tried all these methods and including boiling and steaming each one.   Despite it's flammability, I have also resorted to nail polish and no luck.  Any other suggestions? I'm a neat freak and prefer my kitchen looking clean.  Thank you 
Heat a reasonably damp, but not dripping / soaking, kitchen cloth in the microwave for a minute. The steam produced will soften most deposits and then a bit of elbow grease and it's done. Do on a daily basis or after each use and the interior, especially the roof, will remain clean. Beware of the cloth when removing it! Use another to hold it as it will of course be at the temperature of steam. 