Preheat your oven to 425ºF.

Heat the butter and avocado oil in a large, deep-sided sauté pan or Dutch oven over medium heat. Add the diced onions and fennel, cooking for about 5 minutes.

Add the diced apple and minced garlic to the pan, cooking for another 5 minutes until everything is tender.

While the apple mixture cooks, make fresh breadcrumbs by pulsing a sourdough slice in a food processor or chopping it finely by hand.

Once the apple mixture is tender, remove the pan from heat and stir in the fresh thyme leaves and breadcrumbs.

Butterfly the pork tenderloin by slicing it lengthwise without cutting through, creating a wider, flatter piece. Lay a piece of plastic wrap over the pork, and using a meat mallet, rolling pin, or the side of a can, pound the meat evenly to 1/2-inch thickness.

Remove the plastic wrap and evenly spread the stuffing mixture over the pork.

Slide pieces of kitchen twine under the pork and tie them tightly, using 5 or so pieces to secure the filling. Trim the loose ends of the twine.

Place the tied pork onto a parchment or foil-lined baking sheet. Brush the sides of the pork with oil and season with salt and pepper.

Roast the pork tenderloin in the preheated oven for 25-26 minutes, or until a meat thermometer reads 135 degrees f. The pork will continue to cook about 5º more while resting.

Let the pork tenderloin rest for at least 10 minutes before slicing. Arrange on a platter or plate and drizzle any juices over the top.