1. Preheat the oven to 375.
2. To a large skillet, melt butter and stir in onion, celery, and mushrooms. Sautee for 5-10 minutes until celery is soft and onion is translucent. Add dried spices and stir with a spatula to combine. Pour in chicken stock and bring to a boil. Once boiling, stir in torn bread pieces and mix with broth, spices, and veggies. Cover and remove from heat for 5 minutes.
3. After 5 minutes, remove the lid and fluff with a fork. This is the stuffing. Transfer the stuffing to a plate and let cool slightly. With a paper towel, carefully wipe out the skillet and set it aside to use in the oven.
4. Cut away and discard the silver skin from the pork tenderloin. Butterfly the pork by cutting a slit down the tenderloin lengthwise, but don't quite slice it all the way through. The 2 sides should remain attached.
5. Open the pork tenderloin flat like a book and cover it with a large sheet of plastic wrap. Lightly pound it to an even, ½-inch thickness. Season with salt and pepper.