Preheat oven to 375°F (190°C).
In a small bowl, mix together the brown sugar, olive oil, garlic, salt, and pepper. Rub this mixture evenly over the pork tenderloin to coat it fully.
Place the tenderloin in a large, oven-safe skillet. Roast in the oven for about 30-40 minutes, or approximately 20 minutes per pound, until the internal temperature reaches 145°F (63°C).
When the pork is done, remove it from the oven. Transfer to a platter, covered loosely with aluminum foil, and let it rest for 10 minutes. This helps the juices redistribute for tender slices.
While the pork rests, prepare the cranberry apple topping. In the same pan used to cook the pork, melt the butter over medium heat.
Add the apple, cranberries, apple cider, brown sugar, apple cider vinegar, cinnamon, and nutmeg. Cook, stirring occasionally, for 12-15 minutes, or until the sauce thickens and can coat the back of a spoon.
Return the pork tenderloin to the skillet, spooning the cranberry apple sauce over top.
Garnish with parsley. Slice and serve.