In a bowl, combine the spicy mustard, garlic, and black pepper. Remove the twine and rub the mixture over the beef and place in the refrigerator to chill.
Line a cutting board with plastic wrap and place a layer of puff pastry dough. Arrange the prosciutto on top of the pastry. Evenly spread the onion mixture over the prosciutto.
Position the beef tenderloin at the bottom of the prosciutto. Fold in the edges of the pastry dough and roll the tenderloin, using the plastic wrap to keep everything tight.
Once chilled, remove from the plastic and place it on a foil-lined baking sheet. Brush the pastry with a beaten egg making a few slits on top. Sprinkle with salt.