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

2. In a large skillet, heat olive oil over medium heat. Add chopped onion and sauté until softened, about 5 minutes.

3. Add minced garlic and finely chopped mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms release their liquid and brown, about 8-10 minutes.

4. Stir in the chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat and let cool slightly.

5. In a large bowl, combine the sautéed mushroom mixture with the mashed sweet potato, breadcrumbs, chopped fresh thyme, rosemary, salt, and black pepper. Mix until well combined. Chill the mixture in the refrigerator for at least 15 minutes to firm up.

6. On a lightly floured surface, roll out the puff pastry sheet to about 1/4-inch thickness, creating a rectangle large enough to enclose your filling.

7. Spoon the chilled vegetable mixture down the center third of the pastry, shaping it into a log. Leave a border on both ends.

8. Fold one side of the puff pastry over the filling, then fold the other side over, overlapping slightly. Press the edges firmly to seal. Trim any excess pastry. Carefully transfer the Wellington, seam-side down, to the prepared baking sheet.

9. If desired, lightly score the top of the pastry with a sharp knife (being careful not to cut all the way through) in a decorative pattern. Brush the entire surface with the beaten egg wash (or plant milk for vegan).

10. Bake for 35-40 minutes, or until the pastry is golden brown and puffed. Let rest for 5-10 minutes before slicing and serving.