If you found this article by searching "how to get baked-on grease off of a cookie sheet," sorry, but you're in for a bit of a bait and switch. Before you get all huffy, I promise this isn't some nefarious scheme, rather, my intentions are kind. Instead of busting out some baking soda and vinegar or Bar Keepers Friend, consider this: that baked-on grease can work in your favor.
Do you go crazy for crisp, deeply-browned roasted vegetables? Do you worship at the altar of the Maillard reaction? If so, you have reason to leave that patina alone. The dark color of those splotches promotes browning and might just be the secret to the best darn roast veg of your life. For this reason, I nearly always reach for the same battered, splotchy half sheet pan when crisp results are what I'm after. One small warning: for the same reason why that well-loved pan works so well on the savory side of things, it is the wrong choice for baking cookies or other delicate applications; the dark surface will accelerate browning on the undersides of your sweet treats.