1. Preheat oven to 400°F. Mist a 9-by-13-inch baking dish with cooking spray. Arrange mushrooms, cavity side up, in dish.
2. Finely chop mushroom stems. Melt butter in a medium skillet over medium-high heat. Add onion, chopped mushroom stems and turkey. Sprinkle with salt and cook, stirring often to break up chunks of turkey, until onions have softened and turkey is no longer pink, about 5 minutes. Transfer turkey mixture to a bowl; let cool 10 minutes, stirring occasionally. Stir in bread crumbs and parsley. Gently fold in goat cheese. Taste and season with salt and pepper.
3. Sprinkle mushroom caps with salt and pepper. Spoon filling mixture into mushrooms, piling in center. Bake until lightly browned on top, about 15 minutes. Let cool slightly before serving.