In a large skillet, heat olive oil over medium heat. Add chopped onion and sauté until softened, about 5 minutes. Add minced garlic and cook for another minute until fragrant.

Add the sliced mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms have released their liquid and browned, about 8-10 minutes.

Stir in the cooked lentils, chopped walnuts, chopped spinach (if using), panko breadcrumbs, fresh thyme, fresh rosemary, and vegetable broth. Season generously with salt and black pepper. Cook for an additional 3-5 minutes, allowing some of the liquid to evaporate. Remove from heat and let the filling cool completely.

Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

Unroll the puff pastry sheet onto the prepared baking sheet. Spoon the cooled filling down the center third of the pastry, shaping it into a log.

Bring one side of the puff pastry over the filling, then bring the other side over to meet it, sealing the edges to form a neat parcel. Trim any excess pastry. You can use extra pastry scraps to create decorative cutouts for the top.

Brush the entire surface of the pastry with the beaten egg wash (or plant-based milk for vegan). Gently score the top of the pastry with a knife, being careful not to cut all the way through, to create a decorative pattern. This also allows steam to escape.

Bake for 40-45 minutes, or until the pastry is golden brown and puffed. Let rest for 10 minutes before slicing and serving.