The old recipe for chuck roast, dry onion soup, and mushroom soup wrapped in aluminum foil now burns on the bottom. Any idea why? I bake at 325.

There are 2 sources of sugar in onion soup mix: sugar and maltodextrin. Even at the low temperature of what is essentially a braise, sugar is happy to burn. I'd suggest filling the mushroom soup can half way with water. Swirl it around to rinse the can, and add that to your mix to dilute the dry soup mix. Leave the oven at 325 for about an hour, then turn it down to 300 to finish. The rule for a braise is: low and slow.

New or different pan, perhaps with a darker bottom? Different oven or same one but maybe the temperature gauge is failing allowing the oven to become hotter? Oven rack is in a lower position within the oven? Try using a lighter colored pan vs a darker one, maybe put a layer or two of parchment or foil or a silicone baking mat between the meat packet and the pan, and check the oven temperature and reduce 25 degrees or more to bring it down to 325 if it is running hot. Do let us know what works for you.