Remove the mushroom stems and finely chop them. Preheat the oven to 350°F.
Heat olive oil in a pan. Add the onion and garlic. Saute for a few minutes, then add the chopped mushroom stems. Cook for 3 minutes.
Transfer the mixture to a bowl. Add the walnuts, dried tomatoes, and parsley. Season with salt and pepper.
Stuff the mushrooms with the filling. Bake for 15 minutes.
For the dressing, combine the olive oil, red wine vinegar, garlic, lemon zest, and parsley. Season with salt and pepper.
Pour the dressing on top of the stuffed mushrooms before serving. Enjoy!