For a newer, gentler stain or build-up form, a classic baking soda and vinegar mixture is a great option. This is our favorite and highly recommended method because it's safe, easy, and you probably already have the ingredients in your cupboard! Just open the oven door, sprinkle a generous amount of baking soda over the glass, and spritz some white vinegar over the baking soda. Let the foamy paste mixture set for about 15-20 minutes, then scrub with a soft sponge. Just rinse and wipe with a damp paper towel until the baking soda is gone.
For a more lumpy mess, you might have to bring a big gun. Of course, using harsh chemicals isn't ideal, especially in/near the utensils where you cook the food - but hey, sometimes it's necessary! Combining a good heavy-duty oven cleaner with the right steel wool pad might do the trick. Make sure to use steel wool, or whatever type is indicated to be safe on glass. The "trick" here is that good oven cleaners often take a step in a direction that many overlook - heating the oven on low heat so that the glass is warm but not hot. This creates an ideal surface for the cleaner to work its magic on - so don't skip this step! Again, be sure to rinse thoroughly and wipe off any remaining cleaner.