Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Gently remove the stems from the mushrooms and set the caps aside. Finely chop the stems and set them aside as well.
Heat olive oil or vegan butter in a skillet over medium heat. Add the chopped mushroom stems and garlic, and sauté for about 3-4 minutes until softened and fragrant.
In a bowl, combine the sautéed stems and garlic, chopped walnuts, breadcrumbs, parsley, nutritional yeast (if using), and thyme or sage. Mix well and season with salt and pepper to taste.
Stuff each mushroom cap with the walnut mixture, pressing it down gently.
Arrange the stuffed mushrooms on the baking sheet. Drizzle a bit more olive oil or vegan butter over the tops of the mushrooms.
Bake for 18-20 minutes until the mushrooms are tender and the filling is golden and crispy on top.
Serve hot as a side dish or appetizer!