Mix all the marinade ingredients together and pour in a large zipper freezer bag; place tenderloin in the bag, seal and marinate in the fridge overnight.
Drain and discard marinade. Tie the 2 pieces of the tenderloin together with butcher twine or food-safe string to hold it together.
Preheat oven to 375F/190C. Spray a large baking dish (13x9-inch is good) with non-stick spray or line with parchment. Place in the prepared pan.
Bake for 1-1 1/4 hours or until internal temperature is 160F/70C. Remove from oven and let it rest for 10 minutes. Slice in 1-inch/2.5cm slices.
Serve with roasted mushrooms if desired (recipe in notes section)
Notes
While meat is resting, turn oven temperature up to 425F/220C and in the same pan you cooked the tenderloin in, add the mushrooms and toss to coat with the dripping. Roast for 10-15 minutes.If you don't tie up the 2 pieces to form a "log" then reduce cooking time to 40 min (a clarification brought to my attention from a reader)