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Pork tenderloin demands strict adherence to thermal limits because lean muscle fibers seize and dry out rapidly above 145 degrees Fahrenheit. Combining lean meat with a moisture-rich potato and apple stuffing inside a covered casserole creates a humid thermal environment that buffers the protein against direct radiant heat. Coarsely mashed potatoes retain pockets of air and starch granules that absorb rendered pork fats and fruit juices during the one-hour bake at 350 degrees.
Cooking pork alongside high-moisture ingredients requires balancing steam generation and browning. Covering the three-quart casserole loosely with aluminum foil for the duration of the cooking time traps volatile aromatic compounds from the fresh sage, thyme, and hard cider while preventing the exposed meat surface from scorching. Chopped cooking apples release malic acid and pectin as they soften, which naturally tenderizes the surrounding potato matrix and balances the rich dairy fats from the butter.