Question about cleaning cookie pans

[mercuryblues]

8. Always start with the least invasive

I would try putting some water and baking soda in it, then heat it up in the oven.

When you remove it from the oven scrub it with more baking soda. If still not clean add vinegar. Still not clean up it to salt. Then the bar keepers friend. Then oven cleaner.

If none of that does not work to your satisfaction, congratulations! You now have an oven drip pan and have to buy a new cookie sheet.