Vegetable Wellington with Mushrooms and Spinach

1. Preheat the oven to 400°F. Line a baking sheet with parchment paper.

2. In a large sauté pan, melt the butter over medium heat. Add the leeks and celery. Cook until tender, 3 to 4 minutes.

3. Stir in the mushrooms and continue to cook until they have browned and lost their moisture (they will shrink significantly), 8 to 10 minutes.

4. Add the garlic and sauté 1 minute, until fragrant. Add the spinach in batches, stirring until it wilts completely.

5. Stir in the parsley, rosemary and thyme, and season with salt and pepper to taste. Set aside to cool. (The filling can be made up to two days in advance and refrigerated in an airtight container.)

6. On a lightly floured surface, gently unroll one sheet of puff pastry to flatten it. Using a rolling pin, roll the dough out to about ⅛-inch thick and then transfer it to the prepared baking sheet.

7. Arrange the filling in an even layer in the center of the puff pastry, leaving 1 inch uncovered around the perimeter. Using a pastry brush, coat the exposed edges with water.

8. On a lightly floured surface, gently unfold and roll out the second sheet of puff pastry to about ⅛-inch thick. Carefully lay it over the filling and then seal the edges by pressing with your fingers.

9. If the edges don’t match up, use a knife to trim away any excess.Then crimp the edges with a fork.

10. In a small bowl, whisk the egg with 1 tablespoon water to combine. Brush the egg wash evenly over the surface of the puff pastry.

11. Using a paring knife, cut diagonal lines about 1 inch apart all the way across the puff pastry. Rotate the pan 45 degrees and then repeat this process, making diagonal lines going the other way to create a crosshatch pattern.

12. Sprinkle flaky salt over the surface of the puff pastry and then transfer the pan to the oven and bake for 40 to 50 minutes until golden brown. Let cool at least 15 minutes before slicing and serving. Serve warm.