If you want it to be chemical free, use a paste of baking soda and water. Spread it all over the inside of the oven and let it sit overnight. Wipe off in the morning and repeat if there are really bad spots. I like to use vinegar and water for the final rinse to make it sparkle. Wishing you the best.
I agree with Linda! That is exactly what I use on my oven when I have a nasty, stuck mess. Works great!