Directions

1. Preheat the oven to 425°F.
2. In a small dish, mix the rosemary, thyme, garlic, olive oil, salt, and pepper.
3. Heat oil in a large skillet over medium-high. Add onion, garlic, and mushrooms. Season with salt. Cook, stirring occasionally, until mushrooms are browned, 10 to 12 minutes. Add mustard and continue to cook for about 2 minutes. Let mixture cool completely.
4. Cut a slit down length of each tenderloin, deep enough to reach center, and season with salt. Pack mushroom mixture evenly into slits, then reshape tenderloins into logs with your hands, enclosing filling as much as possible.
5. Unfold pastry on a lightly floured work surface. Roll out to fit around tenderloin. Center a tenderloin on top of dough, stuffed side down. Coat with herb rub.
6. Whisk egg with 1 teaspoon water; brush borders with egg wash. Lift long sides of each dough up and over pork, stretching slightly as necessary until edges meet and trim any extra dough off. Fold short sides up over pork and flip so seams face down. Brush all over with egg wash; cut a few 1-inch vents in tops. Refrigerate 30 minutes.
7. Bake until pastry is puffed and golden brown and a thermometer inserted into centers of pork registers 140°F, 20 to 25 minutes. Transfer Wellingtons to a wire rack; let cool 15 minutes.