Stuffed Mushrooms

Name: Stuffed Mushrooms
Ingredients: 3 slices firm white sandwich bread; 2 Tbsp. extra-virgin olive oil; ½ tsp. kosher salt; ¼ tsp. black pepper; 20 large (2–2½ in diameter) white mushrooms (1 lb.); 2 Tbsp. unsalted butter; 1 medium onion, finely chopped (1 cup); ¼ cup finely chopped celery; 2 garlic cloves, finely chopped; ½ tsp. dried oregano, crumbled; 1 oz. finely grated Parmigiano-Reggiano (½ cup); ¼ cup chopped fresh parsley