For the puff pastry:
- 1 1/2 cups all-purpose flour
- 1/2 teaspoon salt
- 15 tablespoons cold unsalted butter, cut into 1/2-inch cubes
- 1/2 cup ice water
For the Beef Wellington:
- 2 1/2 to 3 lb center-cut beef Tenderloin
- Salt and pepper, to taste
- 2 tablespoons canola or vegetable oil
- 1/4 cup English or Dijon mustard
- 1 tablespoon butter
- 1/2 cup minced yellow onion
- 2 lb cremini mushrooms, minced
- 5 cloves garlic, minced
- 2 eggs, beaten
- Make the pastry: In a large bowl, mix flour and salt with whisk or fork until combined. Using a pastry blender or two sharp knives, cut cold butter into flour mixture until butter pieces are the size of small peas. Pour in cold water and stir with a fork until mixture just comes together (it’ll be pretty rough and shaggy; this is OK).
- Turn dough onto a well-floured work surface. Gently pat dough into a square. Sprinkle top lightly with flour. 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. Fold dough into a letter one final time; wrap tightly in plastic wrap. Refrigerate at least 1 hour up to overnight.
- Make the Beef Wellington: Bring beef Tenderloin to room temperature. If necessary, tie tenderloin into a uniform log shape. Season liberally on all sides with salt and pepper.
- Heat a large cast-iron skillet over medium heat until hot. Add canola oil. Once hot, sear tenderloin on all sides and both ends 3 minutes each until dark brown all around. Transfer to cutting board.
- Remove ties. 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, mushrooms, onions and garlic. Cook, stirring occasionally, 25-30 minutes until all liquid evaporates and mixture cooks down significantly, about one-third to half its original amount. The mixture will be thick and pasty when it’s done.
- Season with salt and pepper. Transfer to a plate to cool.
- 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 oven to 400°F. Line a large rimmed baking sheet with parchment paper; spray with cooking spray.
- 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. Carefully 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 liberally with beaten eggs, then decorate pastry if desired (with extra pastry, fork tines, a knife, etc.). Sprinkle with salt. Bake 35-40 minutes until an instant-read thermometer inserted in center of tenderloin reads 140°F. Let rest 15 minutes before slicing.