Preheat the oven to 400°F (200°C). Line a baking sheet with parchment.

Heat the olive oil in a skillet over medium heat. Add the diced onion and garlic. Sauté until the onion is translucent and soft, about 4–5 minutes.

Add the finely chopped mushrooms. Cook, stirring occasionally, until the mushrooms release their liquid and it evaporates — about 8–10 minutes. Season with salt and pepper.

Transfer the mushroom mixture to a bowl. Add the ground beef, pâté (if using), and Dijon mustard. Mix until evenly combined but don’t overwork the meat.

On a lightly floured surface, roll the puff pastry to smooth seams. Spoon the beef mixture into the center, shaping it into a log or oval, leaving a 1-inch border.

Fold the pastry over the filling and press to seal the edges. Trim any excess pastry.

Place the wrapped Wellington seam-side down on the prepared baking sheet. Brush the top with beaten egg for a glossy finish.

Bake for 25–30 minutes, until the pastry is deep golden brown and the interior reaches a safe temperature of at least 160°F (71°C).

Let the Wellington rest 5–10 minutes before slicing. This helps the filling set and keeps slices tidy.