I have a stainless steel toaster oven. But the baking pans that came with the oven
Get discolored very quickly, some kind of oxidation. The discoloration cannot be washed off. I guess nothing can be done. See picture.

Have you tried using SOS or Brillo pads on them?

This may not be the safest way, so you might want to put on some extra safety gear, but I use an SOS pad with comet and it gets anything and everything off anything stainless steel. I used to use the steel wool, but it left big scratches, but I find the plastic brillo or the SOS pads don't scratch as bad.

I put about 1/4 cup of baking soda in a small glass bowl and squirt in hydrogen peroxide until it makes a nice paste. Just rub it on your pans and do a little scrubbing (shouldn't take much though) and you should be able to notice a HUGE difference! :)

I got something to help you; it's called baking soda, vinegar mixes

How To Clean Burnt Pans With Vinegar and Baking Soda | Apartment Therapy

Also you try barkeeper's friend. Get it at target