These delicious stuffed mushrooms make a fabulous entrée. The hazelnuts are optional but add a nice crunch. To serve as an appetizer, cut each mushroom into 4 pie-shaped pieces.
Preheat oven to 450F. Line two shallow roasting pans or baking sheets with aluminum foil.
Gently wipe mushroom caps with damp towel. Use paring knife to pry out any bits of stems; chop stems finely.
Lightly brush tops of caps with oil. Set caps, gill sides up, in prepared pans. Sprinkle lightly with salt.
Combine chopped stems, breadcrumbs, pesto, lemon zest, vinegar and hazelnuts, if using. Spoon about 2 rounded tablespoons of topping into each mushroom.
Roast mushrooms on top rack 5 minutes. Switch to bottom rack. Continue roasting until tender, 10 to 15 minutes total. Sprinkle grated cheese liberally on each mushroom, and garnish with parsley. Serve warm or at room temperature.