For the Italian Sausage Stuffing:

Add water to a cold skillet. Add the sausage and turn heat to medium. Cook, crumbling sausage until sausage is cooked through and the water has evaporated. Add the apple and onion and continue to cook until softened.

Add bread crumbs and toss to combine. If too dry, add a little wine, apple cider or broth (whichever you are using in the rest of the recipe.) The breadcrumbs should be moist, not wet. Season with the spice rub if desired. Taste and add salt and pepper as needed. Set aside to cool.

For the Tenderloin & Apple & Onion Compote:

Heat the oven to 425°F.

Slice the tenderloin, lengthwise into one flat piece: Cut about an inch into the tenderloin, then using small slices and turning the tenderloin bit by bit, keeping the outside portion as even as you can, cut the tenderloin at about 3/4 inch to one inch, until it lays flat. (Basically, cut it at a spiral.) Place between two pieces of plastic wrap cut side up and pound gently until; it reaches a thickness of about 1/2″.

Rub the spice mixture on both sides of the pork and season generously with salt and pepper. Spread the stuffing mixture down the center of the cut side of the tenderloin, leaving a generous inch and a half along one of the long edges bare. Roll tightly, from the long end with the stuffing spread to the edge towards the bare edge. Tie with butcher twine at 1-inch intervals.

Heat the remaining 2 tablespoons olive oil in an ovenproof 12-inch skillet over medium-high heat. Brown the pork on all sides until golden, about 5 to 6 minutes. Transfer to the oven (a foil-lined sheet tray works well) and roast until an instant-read thermometer inserted in the thickest part of a tenderloin reads 145 to 160 degrees F, 10 to 15 minutes. Transfer the pork to a cutting board and tent with foil.

In the meantime, deglaze the skillet with the wine. When nearly evaporated, add the apples and onions and cook until nearly soft but not falling apart. Season with salt and pepper and more Fennel Spice Mix if desired. Add a scant teaspoon red wine vinegar or sprinkle with a little red wine vinegar to taste.

Slice the pork into medallions and serve with the Apple & Onion Compote.