Follow these simple steps to create a delicious Mushroom Wellington that will impress everyone at the dinner table:

1. Preheat Oven: Start by preheating your oven to 400°F (200°C). This ensures the pastry bakes perfectly.
2. Sauté Vegetables: In a large skillet, heat olive oil over medium heat. Add the onions and garlic and sauté until golden brown.
3. Cook Mushrooms: Add the mixed mushrooms to the skillet. Cook until they are soft and most of the moisture has evaporated, about 8-10 minutes.
4. Add Spinach and Nuts: Stir in the spinach and walnuts (if using) until the spinach is wilted.
5. Flavor Mixture: Pour in the soy sauce and add fresh thyme, parsley, salt, and pepper. Mix well and remove from heat. Allow the mixture to cool.
6. Combine with Breadcrumbs: Once cooled, add the breadcrumbs to the mushroom mixture. This will help bind it together and absorb any excess moisture.
7. Prepare Pastry: On a floured surface, roll out the puff pastry into a rectangle. Ensure it’s large enough to wrap the filling securely.
8. Fill Pastry: Spoon the mushroom filling in the center of the pastry, shaping it into a log.
9. Wrap and Seal: Fold the pastry over the filling, sealing the edges with a fork to ensure it stays together.
10. Egg Wash: Beat the egg and brush it over the top of the pastry for a beautiful golden color.
11. Bake: Place the wrapped Wellington on a parchment-lined baking tray and bake in the preheated oven for 40-45 minutes, or until golden brown.
12. Cool and Slice: Allow the Mushroom Wellington to cool for about 10 minutes before slicing. Serve warm.