Preheat the oven to 350°F. Heat 1 tablespoon of the olive oil in a large skillet over medium-high heat. Add the onions and sauté until they begin to soften, about 5 minutes. Add the spinach and sauté for a couple of minutes until the moisture evaporates. Remove from heat and let cool.
Cut through the pork tenderloin lengthwise to within ½-inch of cutting all the way through. Open up the pork like a book and pound out each side until it’s about ¼-inch thick. Distribute the spinach and onion mix along with the feta over the tenderloin and salt and pepper to taste.
Roll the tenderloin back together, and then wrap the tenderloin with the prosciutto. Once wrapped with the prosciutto, unfold your puff pastry on a flowered countertop and roll your tenderloin around it. Brush on an egg wash.
It is now ready for the oven. I would suggest using a pan with grate on it, that allows for dripping into the pan without getting the bottom of your tenderloin soggy. Roast the pork until a thermometer inserted into thickest part registers 150°F, about 35 minutes.
Many times pork tenderloin's come in a package of two tenderloins. If this is the case, I always just make two of these, they taste great the next day also.