Prepare tenderloins, trimming all silver skin and the thin tail at the end (reserve tail end for another use). Butterfly them for stuffing, making a 1-inch deep incision down the length of each - do not cut all the way through. Stuff each tenderloin with half the Boursin. Roll tenderloins in prosciutto. To do this, lay half the prosciutto slices in a single layer, overlapping them slightly. Place stuffed tenderloin at the base of the prosciutto and roll to cover. Do the same thing with the second tenderloin. Sear in 1 tbsp. olive oil in a large skillet over medium-high heat until prosciutto is brown and crisp on all sides, 5-8 minutes. Chill thoroughly. Thaw two sheets puff pastry and roll each to a 16 x 12 inch rectangle. Wrap pork and transfer to a parchment-lined baking sheet. Combine egg and water and brush wrappers with mixture. Then decorate with pastry vines and cutouts (leaves/stems). Cover with plastic; chill 1 hour or up to 24 hours. Preheat oven to 400 degrees with rack in lower third. Brush with more egg wash for good browning; bake tenderloins 30-35 minutes or until golden, to about 170 degrees. Let rest 5 minutes before slicing. To serve, trim off the ends (they're doughy), and then slice into 1 1/2 inch thick pieces.