Lightly flour a work surface and roll the puff pastry sheet into a rectangle roughly 12 x 14 inches (if frozen, thaw fully in refrigerator before rolling). Keep chilled until ready to fill.

Once filled (see filling steps), place the filling log down the center of the pastry, brush edges with egg wash, fold pastry over to encase the filling and press seam to seal. Trim excess pastry and tuck ends under to form a neat loaf.

Brush entire Wellington with egg wash. Optionally score top with a sharp knife for decoration, being careful not to cut through. Chill assembled Wellington in refrigerator for 15–20 minutes to firm up before baking.

Preheat oven to 400°F (200°C). Place the chilled Wellington on a parchment-lined baking sheet and bake 25–30 minutes until pastry is puffed and deep golden. If pastry is browning too quickly, loosely tent with foil for the last 5–10 minutes.

Remove from oven and rest 10 minutes before slicing. Garnish with chopped parsley and serve with mixed greens.

Vegetable filling

Heat a large sauté pan over medium-high heat. Add 1 tbs olive oil and 1 tbs butter. When butter is foaming, add the sliced leeks and a pinch of salt. Cook, stirring occasionally, until leeks are soft and translucent, 5–7 minutes.

Add the remaining 1 tbs olive oil, then add the finely chopped mushrooms. Spread mushrooms in an even layer and let them sit undisturbed for 2–3 minutes to brown, then stir and continue to cook until most of their moisture has evaporated and they are golden, about 6–8 minutes total.

Add the minced garlic and thyme; cook 1 minute until fragrant.

Add the chopped spinach in batches, stirring until wilted between additions. Once all spinach is wilted, cook for another 2 minutes to drive off excess moisture.

Stir in the chopped chestnuts and cook 1–2 minutes to warm through.

Reduce heat to low. Transfer the vegetable mixture to a sieve set over a bowl and press gently with the back of a spoon to drain any excess liquid. Return mixture to a clean bowl and let cool 5 minutes.

Fold in the softened cream cheese, grated Parmesan, lemon zest, salt and black pepper until evenly combined. Taste and adjust seasoning. The filling should be moist but not wet—if too loose, chill 10 minutes to firm slightly.

Form the filling into a compact log shape on a piece of parchment or cling film about 10–12 inches long. Wrap tightly and chill 15 minutes to set the shape before placing on pastry.