DIAGNOSIS The splatter is baked-on grease and food. The stainless steel interior can be cleaned, but the textured 'roof' around the light and vents is tricky.
FIX Heat a bowl of water with a few tablespoons of white vinegar for 3-5 minutes until the cavity is steamy. Let it sit 5 minutes. Use a soft cloth or non-abrasive sponge with the warm vinegar water and a drop of dish soap to wipe. Avoid the metal grill of the vent.
IF THAT DIDN'T WORK For stubborn spots, make a paste of baking soda and water. Apply it directly to the stain, let it sit for 15 minutes, then gently scrub with the soft sponge.
HEADS UP Never use abrasive pads, steel wool, or harsh chemicals like oven cleaner. They will damage the stainless finish and can create fumes next time you cook.