Soak the California walnuts in hot water for 20 minutes, preheat your oven to 400°F, and chop the garlic, onion, carrots, and mushrooms.
Drain the walnuts, rinse them, and add them to your food processor with the rest of the walnut meat ingredients. Pulse until mixture achieves a fine-ground meat texture, scraping sides as needed.
Melt vegan butter on medium-high heat and sauté onions and carrots for 3-4 minutes.
Add mushrooms and walnut meat and sauté for another 2 minutes.
Add the flour and stir to create a light roux.
Add stout beer, reduce heat to medium-low, cover, and cook for 5 minutes.
Uncover and allow filling to cool and thicken while rolling out pastry.
Roll defrosted puff pastry to ¼ inch thickness and cut into 8 pieces for 4 ramekins or 2 pieces for large baking dish.
Line bottom of ramekins or dish with pastry piece, fill with walnut mixture just below top, then top with another pastry piece.
Bake at 400°F for 15-20 minutes until golden brown and flaky.