For the Pork:

1. Preheat the oven to 450°F.
2. In a large oven-safe cast iron skillet or braising pan, heat the olive oil over medium heat.
3. Add the garlic, shallot, and sage and cook for a couple minutes.
4. Add in the spinach, cherries, and a pinch of salt and pepper and cook, stirring frequently, until the spinach is wilted and the excess liquid has evaporated.
5. Remove from the heat and allow to cool slightly while you prep the pork.
6. To butterfly the pork tenderloin, place it on a cutting board and make an incision with a sharp knife lengthwise, making sure to not cut all the way through. Gently continue slicing the pork with shallow, long knife cuts until it is sliced open but not cut in half all the way through. I like to think of it like opening a book – you want to gently ease it open down the center.
7. Using a meat tenderizer or mallet, gently pound out the meat so it is an even thickness. Season the inside with salt and pepper.
8. Place the spinach filling in the center of the pork, spreading it out evenly and leaving a 1-inch border around the edges.
9. Crumble the goat cheese over the spinach mixture and then tightly roll the tenderloin up lengthwise, securing it closed with kitchen twine. Season the outside of the pork with salt and pepper.
10. Place the pork in the same pan used for the filling and then transfer it to the hot oven. Roast for 30-35 minutes, or until the center reads 140°F on a meat thermometer.

For the Pan Sauce:

1. Remove the pork from the pan, place it on a cutting board, and tent with foil. Allow the pork to rest while you make the pan sauce.
2. Place the same pan used for the pork on the stove over medium heat (be careful as it will be very hot from the oven).
3. Deglaze the pan with the wine, scraping up any bits that may have accumulated on the bottom of the pan while in the oven.
4. Stir in the mustard and apple cider and bring the sauce to a simmer. Cook, stirring frequently, until the sauce is reduced and thickened slightly. Stir in the butter until melted. Season, to taste, with salt and pepper.

To Serve:

Slice the tenderloin into 1-inch thick slices, removing the kitchen twine as you go, and plate with mashed potatoes. Spoon the pan sauce over the pork. Garnish with fresh parsley or chives and serve warm.