Sauté Vegetables: Begin by heating a skillet over medium heat. Add finely chopped onions and mushrooms, stirring often until the moisture evaporates and the mixture becomes golden brown, about 10-15 minutes.

Cool Filling: Once caramelized, remove the skillet from heat. Transfer the vegetable filling to a bowl and allow it to cool completely. This step is crucial for preventing a soggy pastry!

Wrap Filling: Take the thawed puff pastry and roll it out on a lightly floured surface. Place the cooled vegetable filling in the center, and fold the edges over to encase the filling completely.

Prepare for Baking: Brush the wrapped pastry with an egg wash for a beautiful golden color. Use a sharp knife to score a few slits on top, creating vents for steam to escape.

Bake: Preheat your oven to 400°F (200°C). Place the Wellington on a baking sheet lined with parchment paper and bake for 25-30 minutes, or until the pastry is puffed and golden brown.