Preheat oven to 400˚F. In an oven-safe large skillet over medium heat, add 2 Tbsp oil with chopped bacon and and cook until browned (3-4 minutes). Add sliced mushrooms and chopped onion and sauté 5 minutes or until soft. 

Season with 1/2 tsp salt, 1/4 tsp black pepper, chopped parsley and minced garlic. Cook another minute stirring constantly then transfer to a plate. 

Remove Silver Skin from Pork Tenderloin. Cut a slit all the way down the long end of your tenderloin, making sure not to cut all the way through. Open tenderloin like a book, cover with a sheet of plastic wrap and pound with the flat side of a meat mallet until 1/2" thick without tearing through the meat. 

Spread mushroom mixture evenly over the surface of the tenderloin, leaving 1/2" at the borders. Roll tightly starting with the long end (hot-dog style) and secure the ends with 6-7 toothpicks. Poke toothpicks through parallel to each other to create a flat cooking surface. Season all over with 1 tsp salt and 1/4 tsp black pepper. 

Heat the same skillet over medium heat and add 2 Tbsp oil. Once oil is hot, place tenderloin in the skillet (toothpick-side-down) and sear about 2 minutes per side (6-8 minutes total). Transfer the skillet with the tenderloin to the oven and bake at 400˚F for 18-20 min or until an instant read thermometer reads 145-150˚F in the thickest portion of the meat.

Transfer to a cutting board, brush with the pan drippings and rest 10 minutes before slicing into rings. Brush with pan drippings for more flavor and garnish with parsley if desired.