Thaw puff pastry per package instructions.
Remove beef tenderloin from refrigerator and bring to room temperature by having it sit for 30 minutes prior to seasoning and searing.
Prepare the mushroom duxelles: add the portabella mushrooms to the food processor and process until wet and smooth. Transfer to a saucepan over medium heat and add a dash of salt and pepper. Cook and stir until the moisture has dissipated (approximately 10 minutes). Remove from heat and set aside.
Generously season the tenderloin with salt and pepper. Add 1 Tbsp olive oil to a large sauté pan and heat over medium high heat. Add the tenderloin and sear on all sides. Remove from heat and pan and place on a cutting board to cool.
Lightly smear the entire tenderloin on all sides with Coleman’s English Mustard.
Layout enough plastic wrap to wrap the entire tenderloin.
Lay out puff pastry on the plastic wrap. Cover puff pastry with proscuitto and then spread mushroom duxelles evenly over proscuitto. Set tenderloin in the middle and carefully pull plastic over to fully wrap the tenderloin and tuck the ends of the puff pastry and fold in like a present. Make sure it is wrapped tightly and then place in the refrigerator for a minimum of twenty minutes or prepare ahead and leave overnight.
Preheat oven to 375 degrees Fahrenheit.
Whisk one large egg for egg wash.
Take tenderloin out of refrigerator and unwrap the plastic, brush egg wash over outside of puff pastry on all sides.
Cook for 45 minutes for a 4 lb tenderloin or internal temperature is 125 degrees Fahrenheit in center for medium rare.
Once removed from oven, let rest for 15 minutes.
