Preheat oven to 375 degrees.
To prepare the beef tenderloin: Cut off the pointed tip of the roast and the larger end - leaving the main "trunk" of the roast for an even thickness. Using a sharp knife, make a cut about 1/2 inch from the bottom and cut along lengthwise. Be careful not to cut all the way through the meat. Roll the roast back as you cut and continue until the log becomes a flat rectangle. Pat dry.
Roughly chop the mushrooms. In a bowl, combine the mushrooms, garlic, spinach, and balsamic vinegar.
Evenly spread the mushroom mixture onto the flattened beef.
Starting at the longer side, roll the beef into a log. Use the kitchen twine to hold the beef log together. Sprinkle salt and pepper on all sides of the roast.
Spray a baking sheet with cooking spray. Place beef roast on the baking sheet. Roast for 60-75 minutes or until the center of the roast is 145 degrees. Remove from the oven and loosely cover with foil. Let stand for 15 minutes before slicing and serving.