Preheat your oven to 180°C (350°F). Clean 250 grams of button mushrooms by wiping them with a damp cloth. Remove the stems and set the caps aside.
In a pan, heat 1 tablespoon of olive oil over medium heat. Add the finely chopped mushroom stems and cook for 3-4 minutes until they release their moisture and become tender.
In a mixing bowl, combine the cooked mushroom stems, 50 grams of finely chopped walnuts, 100 grams of crumbled blue cheese, 2 tablespoons of finely chopped fresh parsley, 1/2 teaspoon of black pepper, and a pinch of salt. Mix well to create the stuffing.
Stuff each mushroom cap with a generous amount of the walnut and blue cheese mixture. Place the stuffed mushrooms on a greased baking tray.
Bake the stuffed mushrooms in the preheated oven for 15-20 minutes, or until the mushrooms are tender and the cheese is melted and bubbly.
Remove from the oven and let them cool for a few minutes before serving. Garnish with additional chopped parsley if desired.