Prepare the mushrooms: Clean the mushrooms with a damp cloth, removing the stems gently. Set the caps aside.

Toast the walnuts: In a dry pan, toast the walnuts over medium heat for about 3 minutes, stirring occasionally. Set aside.

Make the filling: In a bowl, mix the crumbled blue cheese, chopped walnuts, minced garlic, parsley, salt, and pepper. Add a bit of olive oil if the mixture feels dry.

Stuff the mushrooms: Spoon the filling into each mushroom cap, pressing gently to pack it in.

Cook the mushrooms: Heat olive oil in a pan over medium heat. Place the stuffed mushrooms in the pan and cook for about 5-7 minutes until the mushrooms are tender and the filling is golden.

Serve: Drizzle with balsamic vinegar if desired and garnish with extra parsley before serving.