Heat oven to 425 degrees F.
Heat oil in medium-size skillet over medium heat. Add onion and garlic; cook 10 minutes, until softened. Add spinach, salt and pepper; cook until spinach is wilted and liquid evaporates, 3 to 5 minutes. Remove from heat. Add mustard.
Mix rosemary, salt and pepper in small bowl. Rub over pork.
Open pastry sheet on lightly floured work surface. Roll out to at least 12 inches in length. Spoon onion-spinach mixture into 3-inch-wide mound down center. Place pork on top of spinach mixture, folding over tail end if necessary. Wrap pastry over tenderloin to cover, tucking ends under. Make 3 slashes in top of pastry. Brush pastry with egg wash. Transfer to ungreased rimmed baking sheet.
Roast in 425 degree F oven for 35 to 40 minutes or until instant-read thermometer inserted into tenderloin registers 140 degrees F and pastry is golden brown. If pastry begins to get too brown, cover loosely with aluminum foil. Using 2 large spatulas, transfer roast to a wire rack placed over a dish. Let stand 10 minutes. Makes 6 servings.
