STEP 1: PREPARE THE FILLING

Start by heating a tablespoon of olive oil in a large skillet over medium heat. Add the finely chopped onions and cook for about 2-3 minutes, or until softened. Then add the garlic and sauté for another minute, until fragrant. Add the chopped mushrooms to the pan and cook, stirring occasionally, until the mushrooms release their moisture and become tender, about 5-7 minutes. Season the mushroom mixture with salt, pepper, and fresh thyme, and stir in Worcestershire sauce. Let this mixture cook for an additional 2-3 minutes, allowing the flavors to meld together. Once the mushrooms are fully cooked, remove the skillet from the heat and set it aside to cool.

While the mushroom mixture is cooling, combine the ground beef and egg in a large mixing bowl. Add the cooled mushroom mixture to the ground beef, mixing it gently until everything is evenly incorporated. Taste the filling and adjust the seasoning if necessary. If you’re using cheese or mustard, fold them into the mixture at this point.

STEP 2: ASSEMBLE THE WELLINGTON

Preheat your oven to 400°F (200°C). Lay a large piece of parchment paper or plastic wrap on a flat surface, and then unroll your sheet of puff pastry on top of it. Use a rolling pin to gently roll the pastry out, making it slightly larger if needed, so it can fully encase the beef mixture.

Transfer the beef and mushroom filling onto the center of the puff pastry, shaping it into a log shape that’s the same length as the pastry. Be sure not to overstuff the pastry—leave about 1-2 inches of space on each side to allow the pastry to seal properly.

Once your filling is in place, carefully fold the sides of the pastry over the beef mixture, and then fold up the ends, sealing the filling inside. If necessary, use a bit of water along the edges to help the pastry seal tightly. Once it’s fully wrapped, flip the Wellington over so the seam is on the bottom. If desired, you can use a knife to lightly score the top of the pastry in a crisscross pattern, creating a decorative look (this also helps it puff evenly).

STEP 3: BRUSH WITH EGG WASH

Brush the top of the pastry with the beaten egg to create a golden, glossy finish. This will give your Wellington a beautiful, appetizing color once it’s baked. Place the wrapped Wellington on a baking sheet lined with parchment paper.

STEP 4: BAKE THE WELLINGTON

Place the Wellington in the preheated oven and bake for 25-30 minutes, or until the puff pastry is golden and crisp. The internal temperature of the beef should reach 160°F (71°C) for well-done or 145°F (63°C) for medium (depending on your preference). Keep in mind that the beef will continue to cook slightly as it rests.

STEP 5: REST AND SERVE

Once the Wellington is golden and cooked through, remove it from the oven and allow it to rest for 5-10 minutes before slicing. This resting period ensures that the juices stay inside the beef and doesn’t spill out when you cut it.

After resting, slice the Wellington into thick pieces and serve it with your favorite sides.