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Vegetarian Mushroom Wellington with Caramelized Onions and Spinach

Vegetarian mushroom Wellington is filled with deeply golden caramelized onions, browned cremini mushrooms, and wilted spinach, then wrapped in crisp puff pastry. Cool the filling completely before assembling for a flaky pastry exterior and a savory, sliceable center.
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Prep Time 30 minutes
Cook Time 1 hour 32 minutes
resting 10 minutes
Total Time 2 hours 12 minutes
Servings: 6 servings
Course: Dinner
Cuisine: British
Calories: 470
Ingredients Equipment Method Notes

Ingredients
  

For the filling
  • 4 tbsp olive oil divided
  • 1 tbsp unsalted butter
  • 2 yellow onions large, thinly sliced
  • 0.75 tsp kosher salt divided
  • 1.5 lb cremini mushrooms finely chopped
  • 0.5 tsp black pepper
  • 3 cloves garlic minced
  • 1 tbsp fresh thyme leaves
  • 5 oz baby spinach
  • 0.33 cup panko breadcrumbs
  • 1 tbsp Dijon mustard
For the Wellington
  • 1 tbsp all-purpose flour for dusting
  • 1 puff pastry sheet, frozen, thawed (from a 17.3-ounce package)
  • 1 egg large
  • 1 tbsp water

Equipment

  • 1 12-inch skillet
  • 1 large rimmed baking sheet

Method
 

Prepare the oven
  1. Heat the oven to 400°F.
  2. Line a large rimmed baking sheet with parchment paper.
Make the filling
  1. Heat 2 tablespoons of the olive oil and the butter in a 12-inch skillet over medium heat.
  2. Add the onions and 1/4 teaspoon of the salt; cook over medium-low heat for 30-35 minutes, stirring often, until deeply golden and very soft.
  3. Transfer the onions to a bowl.
  4. Add 1 tablespoon of the remaining oil to the skillet and cook half the mushrooms for 8-10 minutes, until their released liquid evaporates and they begin to brown.
  5. Transfer the mushrooms to the bowl.
  6. Add the last tablespoon of oil and cook the remaining mushrooms the same way, seasoning both batches with the remaining salt and the pepper.
  7. Add the garlic and thyme to the skillet and cook for 1 minute.
  8. Return the onions and mushrooms to the skillet.
  9. Stir in the spinach in handfuls and cook until wilted and the pan is dry, about 2-3 minutes.
  10. Remove the skillet from the heat and stir in the panko and Dijon.
  11. Spread the filling in a shallow bowl and cool completely.
Assemble and bake
  1. Beat the egg and water together in a small bowl to make an egg wash.
  2. Dust the work surface with the flour and roll the pastry into a 12x14-inch rectangle.
  3. Spoon the cooled filling into a 10x3-inch log down the center.
  4. Fold the long sides over the filling and brush the edge with egg wash to seal.
  5. Fold the ends under and place seam-side down on the prepared baking sheet.
  6. Brush the top with egg wash and cut a few shallow slits in the pastry.
  7. Bake for 35-40 minutes, until the pastry is deeply golden and crisp.
  8. Rest on the baking sheet for 10 minutes before slicing.

Notes

Cool the filling completely before wrapping so steam does not soften the pastry. Refrigerate leftovers in an airtight container for up to 3 days; freeze leftovers for up to 2 months. For a gluten-free variation, use gluten-free puff pastry, panko, and flour.