1 pound mushrooms
6 tablespoons butter, melted
½ cup walnuts, lightly toasted and finely chopped
½ cup Italian parsley leaves, chopped
1 medium onion, finely chopped
1 cup sharp cheddar cheese, finely chopped
½ cup fresh bread crumbs
Remove stems from the mushrooms. Finely chop the mushroom stems. Gently brush the melted butter over mushroom caps and arrange on a large, parchment covered baking sheet.
To make the filling:
Sauté the chopped onion and mushroom stems in the remaining butter, until soft, in a large skillet set over medium heat. Sprinkle with a pinch of salt. Allow the mixture to cool slightly.
Stir in the remaining ingredients (walnuts, parsley, bread crumbs and cheese).
Gently mound the filling in each mushroom cap.
Do ahead tip: At this point, the filled mushrooms may be covered and chilled up to four hours.
Bake mushrooms at 350°F for 20 minutes. Serve warm.