Sauté celery, onion, carrot, garlic and mushrooms in a nonstick skillet with a bit of water or oil until soft. Add tomato paste and continue to sauté until vegetables are slightly caramelized and excess liquid has cooked off.
Combine vegetable mixture with lentils, soaked seeds, Worcestershire sauce, poultry spice, flax "egg," and nutritional yeast. Season with salt and pepper to taste.
Stir in breadcrumbs and flour until a firm but not crumbly dough forms. Dough should stick together and hold its shape when pressed in your palm.
Preheat oven to 425°F. Line a 9x5" loaf pan with parchment and then arrange one sheet of puff pastry on the bottom and up the sides of the pan. (You can also assemble the loaf on a baking tray.)
Press the lentil mixture into the loaf pan. Spread on a thin layer of cranberry sauce.
Top with the remaining sheet of puff pastry. Trim overhang, or fold it over and tuck along the edge of the lentil loaf. Cut a few slits to vent steam. Brush lightly with aquafaba, soy milk or vegan butter if desired for a glossy finish.
Bake for 35-40 minutes, until pastry is golden brown and puffed up.
Allow to cool at room temp for about 10 minutes before removing from pan, slicing and serving.