Preheat the oven to 350 degrees. Line a sheet pan with parchment paper. If your puff pastry is frozen, set it out to thaw.

Add the lentils to a pot and cover generously with water. Bring to a simmer and cook until tender, about 15-20 minutes. Drain and add to a large bowl. Mash with a potato masher until half of the lentils are broken up.

Heat the oil in a large skillet over medium-high heat. Add in the mushrooms and cook, stirring occasionally, until browned, about 10-12 minutes. Add in the onion, celery, and cook until softened, about 5 minutes. Add in the garlic and cook for another 60 seconds. Stir in the salt, pepper, sage, thyme, tomato pasta and cook until fragrant, about 60 seconds. Remove from the heat.

To the bowl with the lentils, stir in the miso, mustard, Worcestershire sauce, and flax mixture. Stir in the vegetable mixture and the walnuts.

Roll out the puff pastry into a 9x13 inch rectangle. Spoon the filling into the center of the puff pastry, packing it into a tight log with your hands. Fold the long sides over the middle. On the ends, cut out the overlapping puff pastry to prevent it from getting doughy (see photos in blog post). Fold the short ends up over the log, sealing in the filling. Gently pick up and place the wellington seam side down on your baking sheet.

Score the top of the wellington with shallow cuts in a hatch pattern. Brush the top and sides with vegan egg wash and bake until golden brown, 35-45 minutes.