Combine the ground flax and water in a small bowl. Set aside for 10 min. It will thicken and gel up a bit.
In a large pan with a lid, saute the onions, carrots, celery and mushrooms in 1 tbsp oil until tender, approximately 5 minutes.
Add garlic, cook for 1 minute. Stir in tomato paste, sage, thyme and rosemary, chopped parsley, cook until fragrant.
Add the lentils, broth, Worcestershire sauce, salt and pepper. Bring to a boil, cover with lid, reduce heat to medium-low to maintain a simmer.
Cook approximately 20 to 25 minutes, or until the lentils are tender and have absorbed the liquid.
Mash the lentil mixture a few times.
Mix in frozen peas, chopped walnuts, flaxseed mixture and breadcrumbs. Let the lentil mixture cool to room temperature.
Allow puff pastry sheet to thaw on a floured surface. Once pliable, roll out slightly to create a 10" x 15" rectangle, then transfer to baking sheet lined with parchment paper.
Place filling onto pastry leaving 1.5" at each end. Brush edges of pastry with plant milk. Roll up pastry tucking in ends. Place wellington in the fridge
Preheat oven to 200C.
Score any pattern you like into the pastry. Brush entire pastry with a little plant milk or melted butter.