Season the beef tenderloin with sea salt and black pepper on all sides.
Heat olive oil in a skillet over high heat and sear the beef for 1-2 minutes per side until browned but rare inside; remove and brush with Dijon mustard.
In the same skillet, wilt the spinach quickly, squeeze out all moisture, and chop finely, mixing with minced chives and parsley.
Lay a piece of plastic wrap on a flat surface, arrange prosciutto slices in an overlapping rectangle, and spread the herb-spinach mixture over the top.
Place the beef in the center, roll tightly using the plastic wrap to form a log, and chill in the refrigerator for 15 minutes.
Roll out the puff pastry on a floured surface, unwrap the beef, and place it in the center.
Fold the pastry over the beef, sealing the edges with a bit of beaten egg, and place seam-side down on a parchment-lined baking sheet.
Brush the entire pastry with the remaining beaten egg and bake at 400°F (200°C) for 20-25 minutes until the pastry is golden brown and the beef reaches your desired doneness.