1 1/2 cups all-purpose flour
1/2 tsp salt
15 tBsp cold unsalted butter, cubed
1/2 cup ice water
1 to 1 1/2 kg center-cut beef tenderloin
salt and pepper
2 tBsp vegetable oil
1/4 cup Dijon mustard
1 tBsp butter
1/2 cup minced onion
1 kg minced mushrooms
5 cloves garlic, minced
2 eggs, beaten
Pastry: In a food processor, blend the flour, salt, butter, and water until mixture just comes together (rough and shaggy is ok). Flour a rolling pin and roll out dough to a thin, long rectangle. Fold down top of dough, then fold up bottom of dough, like a business letter; rotate dough 90 degrees. Sprinkle top lightly again with flour, then roll out again to a long, thin rectangle. Fold again like a business letter. Rotate 90 degrees. Repeat rolling, folding and rotating process 6-8 times. Dough will be smooth and have ribbons of butter throughout when it’s done. Wrap tightly in cling wrap and refrigerate for 1 hour.
Beef Wellington: Bring beef Tenderloin to room temperature. Season on all sides with salt and pepper.
Heat a large cast-iron skillet with an oil over medium heat. Once hot, sear tenderloin on all sides and both ends 3 minutes each until dark brown all around. Transfer to cutting board. While tenderloin is still warm, brush mustard all over meat. Let meat rest while making the mushroom mixture.
Heat same skillet over medium high heat until hot. Add butter, minced mushrooms, onions and garlic. Cook, stirring occasionally, 15-20 minutes until the mixture cooks down significantly, about 1/3 to half its original amount. The mixture will be thick and pasty when it’s done. Season with salt and let it cool down.
Place a large layer of plastic wrap on a clean work surface. Spread cooled mushroom mixture in a rectangle the same length and about twice the width of the tenderloin. Place tenderloin on bottom half of mushroom mixture. Carefully roll up tenderloin in plastic wrap so it is completely encased in mushroom mixture. Seal in plastic wrap and tie ends to hold Wellington’s shape. Refrigerate 15 minutes.
Heat the oven to 200°C. Line a baking tray with parchment paper; spray with an oil.
Remove puff pastry from refrigerator. On lightly floured surface, roll out pastry to 1/4-inch thick rectangle about 2 inches longer than length of tenderloin, and about twice its width. Unwrap tenderloin onto bottom half of puff pastry, ensuring that mushroom mixture remains intact around tenderloin. Wrap pastry tightly around tenderloin until ends meet. Trim any extra pastry, then pinch seam to seal tightly. Fold over and pinch ends to seal so tenderloin is completely encased in pastry. Wrap tightly in plastic wrap. Refrigerate 15 minutes.
Unwrap and place Wellington on prepared baking sheet. Brush all over with beaten egg. Sprinkle with salt.
Bake 35-40 minutes until an instant-read thermometer inserted in center of tenderloin reads 70°C. Let rest 15-20 minutes before slicing.