- Preheat your oven to 375°F (190°C). 
- Clean the mushroom caps with a damp cloth and remove the stems. Set caps aside and finely chop the stems. 
- Heat olive oil in a skillet over medium heat. Add the chopped stems, onion, and garlic. Cook until soft and fragrant—about 5 minutes. 
- Stir in the breadcrumbs, walnuts, parsley, salt, pepper, and nutritional yeast if using. Cook another 2 minutes. 
- Scoop the mixture into the mushroom caps, pressing down gently. 
- Arrange stuffed mushrooms on a baking sheet lined with parchment paper. 
- Bake for 20 minutes, or until the tops are golden and the mushrooms are tender. 
- Serve warm with an extra sprinkle of parsley, if you’re feeling fancy.