You can try Barkeeper's Friend and a plastic scrubby but to be honest, after trying many tricks, I haven't found anything that truly works so I've given up.
Mix baking soda and hydrogen peroxide into a paste.
Spread mixture across the top of the pan.
Let sit for 2-3 hours.
Wipe mixture away with a paper towel and water.
If necessary, scrub any remaining sections.
Hello. Bar Keepers Friend does the trick for me. It makes them look brand new again.
Brillo or put inside a self clean oven
To save yourself the aggravation of repeated scrubbings, purchase parchment paper, which can be reused, multiple times. Simply wipe it with a damp rag, between uses.