1. Tear bread into pieces; place in food processor. Process 30 seconds, or until crumbs are formed. Transfer to small bowl; set aside.
2. Remove stems from mushrooms; reserve caps. Place mushroom stems, celery, onion, and garlic in food processor; process using on/off pulses until vegetables are finely chopped.
3. Spray large skillet with nonstick cooking spray. Add vegetable mixture; cook and stir over medium heat 5 minutes, or until onion is tender. Remove to bowl. Stir in bread crumbs, Worcestershire sauce, marjoram, and red pepper.
4. Fill mushroom caps evenly with mixture, pressing down firmly. Place about 1/2 inch apart in shallow baking pan. Spray tops with cooking spray. Sprinkle with paprika.
5. Preheat oven to 350°F. Bake 15 minutes, or until heated through.
Note: Mushrooms can be stuffed up to one day ahead. Refrigerate filled mushroom caps, covered, until ready to serve. Bake in preheated 300°F oven 20 minutes, or until heated through.