Heat olive oil in a large skillet over medium-high heat. Add diced onion and cook 5 minutes until softened and golden. Add minced garlic and chopped mushrooms, cook 8-10 minutes until mushrooms release moisture and liquid evaporates completely. Stir in fresh spinach and cook 2-3 minutes until wilted. Season with thyme, rosemary, salt, and pepper. Transfer mixture to a fine sieve, press down to drain excess liquid, and cool for 15 minutes.
12 oz mushrooms, 1 medium onion, 3 cloves garlic, 4 cups spinach, 2 tsp fresh thyme, 1 tsp fresh rosemary, 2 tbsp olive oil, to taste salt, to taste black pepper
Roll puff pastry on parchment paper to ⅛ inch thickness (about 12x10 inches). Spread breadcrumbs in a 3-inch strip down the center, leaving 2 inches on each side.
1 sheet puff pastry, 3 tbsp breadcrumbs
Spread goat cheese evenly over breadcrumbs, then top with the cooled vegetable mixture, pressing gently to an even layer.
4 oz goat cheese
Fold one side of pastry over the filling, then fold the other side to overlap slightly. Press edges firmly to seal the Wellington. Flip seam-side down onto a parchment-lined baking sheet. Tuck ends under and press to seal completely.
1 sheet puff pastry
Beat the egg with 1 tablespoon water and brush the egg wash evenly over the entire pastry surface. Score diagonal shallow lines across the top with a sharp knife without piercing through. Chill in the fridge for 10-15 minutes while preheating oven to 400°F (205°C).
1 egg egg
Bake on the center-lower rack for 35-40 minutes until the pastry is deeply golden brown and puffed. Check at 25 minutes; if edges brown too fast, tent with foil. Remove from oven and let rest 10 minutes before slicing. Pastry should be crisp with filling heated through and cheese melted.
