Preheat oven to 425˚F.

Heat an oven proof skillet over medium-high heat. Add the bacon and onions and cook until the bacon is just starting the brown and the onion have become transparent, about 5 – 7 minutes.
NOTE: if you’re using the bacon bits, sauté the onion in 1 tablespoon of oil until they just become transparent. Add the bacon bits after the mixture has been transferred to the bowl in Step 5.

Add the garlic and cook until it becomes fragrant, about 30 seconds.

Add the mushrooms, season with 1/2 teaspoon of salt and 1/4 teaspoon of pepper, stir and cook until the mushrooms start brown around the edges, about 3 – 5 minutes.

Add the spinach, stirring until it has wilted, about 3 minutes. Transfer to a bowl, set aside.
NOTE: if using frozen spinach, stir until it’s evenly distributed throughout the mixture.

Remove any excess fat and the silverskin from the pork.

Butterfly the tenderloin by slicing lengthwise and open it up like a book. Be careful not to cut all the way through.

With a meat mallet, rolling pin, or a heavy skillet, pound the meat to an even thickness, about 1/2-inch.

Season the inside of the tenderloin with 1 teaspoon of salt, 1/4 teaspoon of pepper, and the thyme.

Spread the spinach filling over the pork edge to edge, leaving about 1-inch bare on one long side. Sprinkle the Parmesan over the filling.

Starting at the long edge with the filling, roll the tenderloin toward the bare long edge. Secure with cooking twine.

Heat the oil in the oven-proof skillet. Brown the three non-seam sides of the tenderloin, 2 – 3 minutes per side.

Transfer the skillet to the oven and cook until an instant read thermometer reads 145˚F when inserted into the center of the pork tenderloin.

Allow to rest 3 – 5 minutes, slice and serve.