If you found this article by searching "how to get baked-on grease off of a baking tray," 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 whipping out the baking soda and vinegar, consider this: that baked-on grease can work in your favour.
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 colour of those splotches promotes browning and might just be the secret to the best roast veg of your life. For this reason, I nearly always reach for the same battered, splotchy baking tray 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 savoury side of things, it is the wrong choice for baking biscuits or other delicate applications; the dark surface will accelerate browning on the undersides of your sweet treats.