Preheat the oven to 200 degrees Celsius (400 degrees F)
In an oven proof skillet cook the bacon in olive oil until crisp. Add in the diced onion, garlic and mushrooms, sprinkle with salt and cook for about 8 minutes until the mushrooms have softened.
Add in the spinach and parsley and leave to cook for a few minutes until the spinach has wilted. Stir in the pine nuts, remove the pan from the heat and set aside.
To prepare the tenderloin, start by cutting the silver skin off the top of the tenderloin (this makes the meat quite chewy). Cut lengthwise down the side of the tenderloin through the middle horizontally to within one-half inch of the other side. Open the tenderloin flat like a book and lay it open on a cutting board. Place plastic wrap over the meat and then beat it down with a mallet or rolling pin until approx 1/2 an inch thick.
Spoon the mushroom and bacon mixture evenly over the tenderloin and then roll it up, pressing down to ensure its very tightly wrapped up. Secure the loose end with toothpicks, skewers or alternatively twine (watch the video above for clarification). Season the outside of the tenderloin roll with salt and pepper.
Heat the olive oil in the oven proof skillet and sear the tenderloin on all sides. Once golden in colour, transfer the skillet to the oven and bake for 15-17 minutes until a meat thermometer inserted in the centre reads 145 degrees.
Remove the tenderloin from the oven and place on a cutting board to rest for 5 minutes. Brush the tenderloin with any of the drippings left in the pan before slicing.