1. Prepare the Mushroom Filling

To begin, heat the olive oil in a large skillet over medium heat. Add the chopped onion and garlic and sauté for 3-4 minutes, or until softened and fragrant. The onions should become translucent, and the garlic should be aromatic but not browned.

Add the chopped mushrooms to the skillet and cook them down, stirring occasionally. The mushrooms will release a lot of moisture, so cook them until this liquid evaporates, and the mushrooms start to brown. This process should take about 10-12 minutes. The mushrooms will shrink and become tender, which is essential for the texture of the filling.

Once the mushrooms are browned and most of the moisture is gone, stir in the chopped spinach. Cook for another 2-3 minutes, or until the spinach has wilted. If you’re making the duxelles (optional), you can add the heavy cream and brandy or white wine at this point and simmer the mixture for a few more minutes until thickened.

Season the mushroom mixture with salt, pepper, fresh thyme, and soy sauce or tamari for an added umami boost. You can also add a little bit of Dijon mustard at this point for an extra layer of flavor. Taste and adjust the seasoning as needed.

If desired, add the breadcrumbs to the mixture to help bind the filling together. The breadcrumbs will provide a bit of texture and help absorb any remaining moisture. Set the mushroom mixture aside to cool while you prepare the pastry.

2. Prepare the Puff Pastry

Lay the sheet of puff pastry on a large, lightly floured surface. If your pastry is frozen, make sure it is fully thawed before working with it. Roll out the pastry slightly with a rolling pin, just to smooth out any creases. You want the pastry to be large enough to wrap around the mushroom filling completely.

Once the pastry is prepared, place a piece of parchment paper on a baking sheet to prevent sticking. You will need enough room to place the mushroom filling in the center and fold the pastry around it.

3. Assemble the Wellington

Spoon the cooled mushroom filling onto the center of the puff pastry sheet. Be sure to spread the filling out evenly, leaving enough space around the edges to fold the pastry over. The filling should form a log-like shape in the middle of the pastry.

Fold the sides of the pastry over the mushroom mixture, and then carefully fold the ends to fully enclose the filling. Use a little water to seal the edges of the pastry if necessary. Once the Wellington is sealed, carefully flip it over so the seam side is down on the baking sheet.

Brush the top of the Wellington with the beaten egg to give it a beautiful golden color when baked. If you like, you can also score the top of the pastry lightly with a knife to create a decorative pattern.

4. Bake the Mushroom Wellington

Preheat your oven to 400°F (200°C). Place the prepared Wellington in the oven and bake for 25-30 minutes, or until the puff pastry is golden brown and crispy. The filling should be hot and the pastry should have a nice, flaky texture.

To check for doneness, gently tap the top of the pastry. It should sound hollow, indicating that the pastry is fully cooked. If the pastry is not golden enough, you can bake for an additional 5 minutes, but keep an eye on it to prevent overbaking.

5. Serve and Garnish

Once the Mushroom Wellington is baked to perfection, remove it from the oven and let it cool for about 5 minutes before slicing. This will allow the filling to set slightly and make it easier to cut. Serve the Wellington with a garnish of fresh parsley, if desired, for a pop of color.

Slice the Wellington into generous portions, and enjoy with your favorite sides or a refreshing salad.