Mushroom and Walnut Wellington

Heat the oven to 425°F. Line one baking sheet with parchment. Wash your hands and clean the work surface. Briefly rinse the mushrooms and thyme under running water and dry thoroughly; rinse and dry the spinach unless labeled prewashed. Remove damaged areas. Remove the portobello stems and gently scrape out the dark gills with a spoon, keeping every cap intact. Finely chop the baby bella mushrooms and thyme.

Brush the portobellos with their allocated 1 tablespoon olive oil. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Arrange gill-side down on the lined sheet and roast for 20–25 minutes, until tender and noticeably shrunken. Transfer to a plate, drain away released liquid and blot both sides dry. Leave the caps whole.

Meanwhile, toast the walnuts in the dry skillet over medium heat for 4–6 minutes, stirring frequently, until fragrant and lightly browned. Transfer to a board, cool briefly and chop into small pieces.

Heat the remaining 2 tablespoons olive oil in the skillet over medium-high heat. Add the finely chopped baby bella mushrooms, remaining 3/4 teaspoon salt and remaining 1/4 teaspoon pepper. Cook for 15–20 minutes, stirring frequently, until their liquid has evaporated and the mushrooms begin to brown. The mixture should leave no watery trail when pushed aside. Stir in the thyme for the final minute, then transfer to the mixing bowl.

Add the spinach to the same skillet in batches. Cook over medium heat for 2–4 minutes, turning until just wilted; do not add water. Let it cool enough to handle, then wrap it in the clean towel and squeeze firmly until no more liquid comes out. Chop the squeezed spinach and stir it into the chopped mushrooms with the walnuts and all 3 tablespoons Dijon.

Line a cool baking sheet with fresh parchment. Divide the pastry into two equal 12-ounce portions. Using the measured 2 tablespoons flour to dust the work surface and rolling pin, roll each portion into a rectangle approximately 14 by 12 inches, about 1/8 inch thick. If joining pastry pieces, overlap their edges by 1/2 inch and press firmly before rolling. Work with one rectangle at a time, keeping the other chilled.

Divide the filling into four equal portions. Spread one portion in a 10-by-4-inch strip down the center of the first pastry rectangle, parallel to its 14-inch sides. Arrange three whole portobello caps in a snug row on top, gill-side down. Cover with a second portion of filling, pressing it gently around the caps to create an even mound. Leave the pastry borders clean. Repeat with the second rectangle and remaining filling and caps.

Beat the egg in the small bowl. Brush the pastry edges lightly with a little beaten egg. Bring the long sides up over the filling, overlapping them by about 1 inch, and press firmly to seal. Fold the short ends inward and seal them completely, avoiding thick bunches of dough. Carefully turn each loaf seam-side down onto the prepared sheet, leaving at least 3 inches between them. Refrigerate for 20 minutes.

Brush the tops and sides lightly with the remaining beaten egg, discarding any unused egg. Cut three 1-inch slits through the top pastry of each loaf to release steam, without cutting deeply into the filling.