Start by preheating the oven to 425F and preparing a baking pan with nonstick cooking spray.
In a large skillet, heat the oil over medium heat and cook the onion until tender. Add the garlic and cook for 1 more minute. Add the spinach and cook until wilted and most liquid has evaporated.
Remove from the liquid and stir in the mustard; set aside.
In a small dish, mix the rosemary, salt, and pepper. Rub into the tenderloin, making sure to cover all sides.
Open the thawed pastry sheet onto a lightly floured surface and roll it out until it reaches 12″ in length.
Spoon the spinach mixture into a 3 inch mound down the center.
Carefully place the pork on top of the spinach before folding the dough. Fold the ends first, then wrap and tuck around the tenderloin.
Carefully move the tenderloin into the prepared baking sheet so the spinach mixture is sitting on top, and the sealed ends are on the bottom. Use a knife to make a few slices on top of the pastry sheet for a steam vent.
Brush the egg wash over the dough and bake in the oven for 35-40 minutes, or until the internal temperature reaches 145F, according to a meat thermometer. You do not want to skip the egg wash as it gives a nice golden crust!