Preheat oven to 350 degrees. With a sharp boning knife, trim excess fat from beef tenderloin. Carefully cut down the middle of the tenderloin, not cutting the whole way through, and butterfly the meat, so that if can be unfolded.
Using a meat pounder or mallet, gently pound beef to 1/4" to 1/2' thick so that it forms one large, flat piece. Spread mushroom duxelle evenly over beef, leaving a half-inch clean space around border. Spread marinara sauce on top of duxelle. Layer blanched spinach on top of sauces. Then layer red and yellow peppers on top of spinach.
Starting at one wide end, tightly roll up beef, packing in all the filling as you go. Fold in loose ends to keep it neat and prevent filling from spilling out. Using butcher's twine, tie loops around beef roll every 2-3 inches. Trim excess string with kitchen scissors.
Place tenderloin in an uncovered roasting pan and bake at roast at 350 degrees for around 1 and 1/2 hours, depending on your oven. Using a meat thermometer, check the thickest part of the beef and stop cooking when it reaches your desired temperature Rare: 125°, Medium Rare: 135°, Medium: 145°, Medium Well: 155°, Well Done: 160°.
Remove from oven and allow to rest for 10 minutes. Slice into thin pinwheels and serve with sauce, asparagus, and potatoes. Makes 8-12 servings. Enjoy!