Prepare the Filling

1. Sauté the Vegetables: Heat olive oil in a skillet over medium heat. Add onions and garlic, cooking until softened.
2. Add Mushrooms and Veggies: Add mushrooms, carrot, and celery, and sauté until softened and moisture evaporates.
3. Incorporate Spinach and Seasoning: Stir in spinach, lentils, walnuts, thyme, rosemary, salt, and pepper. Cook until well combined, then remove from heat and let cool.

Assemble the Wellington

1. Prepare Pastry: Roll out the puff pastry on a floured surface to fit the filling.
2. Add Filling: Place the filling in the center of the pastry, shaping it into a log.
3. Wrap: Fold the pastry over the filling, sealing the edges by pressing firmly. Transfer to a lined baking sheet.
4. Brush with Milk: Brush the top with plant-based milk for a golden finish.

Bake the Wellington

1. Bake: Preheat oven to 400°F (200°C). Bake the Wellington for 30-40 minutes, until golden brown.
2. Cool and Slice: Let the Wellington cool for a few minutes before slicing and serving.