Sauté the Mushrooms: Heat olive oil in a skillet over medium heat. Add the onions and cook until softened, about 3 minutes. Add the garlic and cook for another minute.
Add the Herbs: Toss in the chopped mushrooms, rosemary, thyme, sage, salt, and pepper. Cook until the mushrooms are tender and most of the moisture has evaporated, about 10 minutes. Set aside to cool.
Prepare the Pastry: Roll out the puff pastry on a floured surface. Spread a thin layer of Dijon mustard in the center.
Assemble: Layer fresh spinach leaves on top of the mustard, then spoon the mushroom mixture onto the spinach, shaping it into a log. Fold the puff pastry over the filling, sealing the edges to form a tight bundle. Brush with egg wash if desired.
Bake: Place the Wellington on a baking sheet lined with parchment paper. Bake at 400°F (200°C) for 25-30 minutes, or until golden brown.
Serve: Let it cool slightly before slicing. Serve warm with cranberry sauce or a rich mushroom gravy for an extra touch.