Butterflying a tenderloin: Trim the silver skin or connective tissue with a very sharp, thin knife. Place the tenderloin on a cutting board vertically, then slice through the tenderloin lengthwise down the side, about a third of the way through.
Fold that flap open and give it a light pounding using a mallet or the back of a small sauté pan. Turn the cutting board 180° and slice into the tenderloin down the other side, two thirds of the way through, so that it opens up like an accordion or a business letter. Give that side a light pounding, so that the whole tenderloin is flat, rectangular and ready to be stuffed and rolled up. Repeat with other tenderloin.
In a small bowl, combine the honey and mustard.
In a large mixing bowl, add the breadcrumbs, melted butter, spinach, salt, pepper, orange zest and the beaten egg, stir to combine.
Brush the honey mustard mixture on the inside of the butterflied tenderloins. Spread the spinach mixture evenly over the honey mustard, leaving a one-inch border.
Roll the tenderloins back up and place them seam-side down on a nonstick baking sheet. Coat them with EVOO, salt, pepper and the orange juice, pop them into the oven and roast for 15-20 minutes, or until the internal temperature reaches 140-145°F. Transfer the meat to a cutting board and let it rest.
Slice meat and serve with Mini Twice-Baked Potatoes alongside.