Preheat the oven to 375°F.
Heat olive oil in a large pan over medium heat. Add the onion and garlic and sauté until soft. Add the mushrooms, carrot, and celery and cook until the mushrooms release their moisture and the vegetables are tender.
2 cups cremini mushrooms, 1 cup onion, 3 cloves garlic, 1 carrot, 2 tablespoons olive oil, 1 celery stalk
Stir in the cooked lentils, walnuts, mustard, soy sauce, herbes de Provence, bread crumbs, parsley, salt, and pepper. Cook for 5–7 minutes to combine flavors, then remove from heat and let cool completely.
2 cups cooked brown lentils, 1/2 cup breadcrumbs, 1/2 cup walnuts, 1 tablespoon whole grain mustard, 1 tablespoon soy sauce, 1/4 cup fresh parsley, 2 teaspoons herbes de Provence, Salt, Black pepper
Roll out the puff pastry on a lightly floured surface. Spread the mustard in the center, leaving a 2‑inch border on each side. Spoon the lentil mixture over the mustard.
1 sheet puff pastry, 1 tablespoon whole grain mustard
Fold the pastry over the filling and seal the edges, crimping with a fork for venting.
Mix the ground flaxseed and water to make a flax egg. Brush the pastry with the flax mixture. Place the Wellington on a baking sheet and bake for 25–30 minutes, until golden and crisp.
1 tablespoon ground flaxseed mixed with 3 tablespoons water
To make the gravy, whisk together the vegetable broth, nutritional yeast, soy sauce, olive oil, thyme, salt, and black pepper in a small pot. In a separate bowl, mix the cornstarch with water, then stir it into the pot.
1 cup vegetable broth, 2 tablespoons nutritional yeast, 1 tablespoon soy sauce, 1 teaspoon olive oil, 1 teaspoon dried thyme, 1 tablespoon cornstarch, 1 tablespoon water, Black pepper, Salt
Bring to a simmer over medium‑high heat, then reduce to low and cook for 5–10 minutes, stirring occasionally, until thickened. Taste and adjust seasoning if needed. If the gravy becomes too thick, whisk in a little more broth or water.
Serve the Wellington warm with the gravy on the side.