Open the tenderloin, and using a meat mallet, carefully pound the meat just until it is of even thickness.
In a heavy skillet, heat 2 tablespoons of the olive oil and once sizzling add the onion. Cook until softened and then add the garlic Cook an additional minute or two.
Add the mushrooms, and cook for 5 to 8 minutes until softened and beginning to brown.
Add the spinach in parts, stirring it into the onion mixture until it becomes softened.
Add the sun-dried tomatoes, and cook an additional two minutes. Season with salt and pepper.
Place the tenderloin open on a flat surface. Spoon the stuffing mixture evenly over the meat, leaving a 1 inch border all around.
Firmly roll the tenderloin keeping the filling in place.
Secure the tenderloin with butcher’s twine.
Rub the tenderloin with the remaining olive oil, and then rub on the cracked pepper.
Place the tenderloin in a baking pan and cook for about 40 minutes, or until a meat thermometer registers 160 degree F. internal temperature.
While the meat is cooking, place the red wine and beef broth in a heavy saucepan and cook on medium high heat until it has reduced to about 3/4 to 1 cup of liquid. Taste, and season as needed with salt and pepper. Whisk in the butter and keep warm.
Once the tenderloin has reached an internal temperature of 160 degrees F., remove it from the oven, and cover with aluminum foil. Let rest for 10 minutes. Slice the tenderloin into 1 1/2 inch thick slices, and serve with a drizzle of the red wine sauce. Serve.