Preheat oven to 325 degrees F. With a sharp knife, cut the tenderloin down the middle lengthwise without cutting through. Open tenderloin so it lies flat; cover with plastic wrap. Working from the center to edges, lightly pound tenderloin with flat side of meat mallet or rolling pin to a rectangle with an even thickness throughout. Spread Dijon mustard over tenderloin and salt and pepper. Cut roasted red peppers to lay flat and lay evenly over Dijon mustard. Next, layer cheese over roasted red peppers. Finally, spread and even layer of spinach over cheese. Sprinkle with lemon zest and lightly salt and pepper.
Roll the tenderloin up from longest side. Tie up with butchers twine that has been soaking in water.
In small bowl combine oil, thyme, garlic salt and lemon-pepper seasoning. Rub mixture over the outside of the tenderloin. Place roll seam side down on a rack in a shallow baking dish. Roast for about 45 minutes or until the meat thermometer reads 145º F. Let stand 3 minutes. Remove string and slice.