Cook the lentils. Instant Pot: add the lentils to the inner pot along with 2/3 cup (160 mL) water and a pinch or two of kosher salt. Use the pressure cook setting at high pressure for 6 minutes. Allow a natural pressure release (after 10 minutes, manually release any remaining steam). If they're not tender yet, pressure cook for 1-2 additional minutes; 5 minute pressure release. Stovetop: in a small or medium saucepan, add the lentils and cover with a decent amount of water. Bring to a boil, then reduce the heat and cover the pan to maintain a simmer. Simmer, stirring occasionally, for 20 to 30 minutes, or until tender but not mushy. Drain off any excess water. Transfer cooked lentils to a bowl and refrigerate until cooled. Add the mushrooms to a food processor and blitz several times until they’re finely chopped. Or, finely chop with a knife.
Cook the mushrooms. Heat the olive oil in a large frying pan over medium high heat. Add the onions with a tiny pinch of kosher salt and cook for 7 to 10 minutes, stirring occasionally or until softened and golden brown. Add the garlic, mushrooms, thyme, rosemary, paprika and black pepper. Cook until the mushrooms are softened and the liquid has evaporated, about 5 minutes. Then add the kosher salt and cook for one additional minute. Remove the mushrooms from the heat and transfer them to the refrigerator to cool down. Toast and blend the walnuts. While the mushrooms are cooling, toast the walnuts***. Allow to cool slightly. Transfer the toasted walnuts to a food processor and blitz until you have fine crumbs, but don’t over-process or it will start to turn into walnut butter. Transfer the ground walnuts to a bowl.
Preheat the oven to 375°F / 190°C.
Make the mushroom-lentil filling. Add the cooled lentils and mushroom mixture to the food processor, along with the miso paste, nutritional yeast, tomato paste, and lemon juice. Blend until you have a somewhat smooth, slightly sticky mixture that comes together.1. Add in the reserved ground walnuts, bread crumbs, and flour, and blend again until all of the ingredients are well incorporated.2. For a visual of what the consistency should look like, check out the photos in the blog post.3. If your filling is warm (this will be the case if you did not cool the cooked mushrooms), refrigerate it for 15-30 minutes until it is easy to mold together with your hands. Line a baking sheet with parchment paper. Transfer the mushroom filling to the pan and use your hands to form it into a log shape that is roughly 7 inches long and 4 ½ to 5 inches wide (18cm x 11.5-12.5 cm), and no taller than 1 inch. See the photos in the blog post for reference
Bake the filling. Bake the mushroom log in the preheated oven for 35 minutes until it is relatively firm to the touch and brown and slightly crisp on the outside (if testing it with a thermometer, the internal temperature should read between 200-205ºF or 94-96ºC). Allow to cool completely and turn off oven.
Defrost the puff pastry and/or make the gravy. While the log is cooling, place some flour down on a flat work surface and lay the sheet of puff pastry down. Allow to thaw and come to room temperature, about 20-30 minutes. If you are making the gravy, you can get started on that.NOTE: I don't recommend thawing the pastry for more than 30 minutes, as it can get too warm and sticky to roll. Assemble the pastry. Once the puff pastry is thawed, use a rolling pin (or a chilled wine bottle) to roll the pastry into a rectangular shape that is roughly 9 inches wide x 13 inches long (23 cm wide vs. 33 cm wide).
Braid the dough (look at the photos in the blog post): Place the cooled mushroom log in the middle of the pastry sheet so that the top and bottom of the log almost touch the edges of the pastry, widthwise. Start on one end of the pastry and use a paring knife to cut 1 inch/2.5 cm strips about a fourth of the way up, almost but not quite reaching where the mushroom log is. Repeat on the other side. You should have about 9 strips on each side.1. Whisk together the plant-based milk and oil in a small bowl. This is the vegan “egg wash.” Use a pastry brush to brush the egg wash onto each strip of dough.2. Starting at one end, braid a pastry strip on top of the log, alternating each side. When you finish braiding the strips, pinch and tuck the in the edges into the top and bottom of the log, and brush on some of the egg wash to help seal the edges. Lightly prick the dough all over with a fork - the small holes will allow steam to escape during baking, making for more even baking. Carefully transfer the Wellington to a baking sheet lined with parchment paper. Brush the egg wash all over the rest of the pastry (you may not use it all) and then sprinkle with some coarse/flaky sea salt. Refrigerate the pastry for 15-20 minutes to allow the dough to cool back down (baking the puff pastry while it’s warm may cause it to bake unevenly).
Preheat the oven to 375°F / 190°C and line a baking sheet with parchment paper. Remove the pastry from the fridge and bake for 25 minutes. Then increase the oven temperature to 425°F / 218°C and bake for another 10 minutes, or until the top is beautifully golden brown, flaky, and puffed and the internal temperature reaches 200-205ºF or 94-96ºC.
Allow to cool for 10 minutes before slicing through. Serve plain or with the Mushroom Gravy.