Preheat your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until translucent.
Stir in the minced garlic and chopped spinach, cooking for another 2-3 minutes until the spinach wilts.
Remove the skillet from heat and add the breadcrumbs, nutritional yeast, soy sauce, dried thyme, black pepper, and walnuts if using. Mix well until all ingredients are combined.
Fill each portobello mushroom cap generously with the stuffing mixture, pressing it down slightly to pack it in.
Arrange the stuffed mushrooms on a baking sheet lined with parchment paper, and drizzle a little olive oil over the top for added flavor.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is golden brown.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.