1. Preheat oven to 400°.
2. Bring 1 1/2 cups water and butter to a boil in a medium saucepan. Stir in stuffing mix. Cover, remove from heat, and let stand 5 minutes; fluff with a fork. Let stand, uncovered, 10 minutes or until cool enough to handle.
3. While stuffing cools, cook sausage in a small skillet over medium-high heat until browned; stir to crumble. Drain.
4. Coat fingers with cooking spray. Press about 1/4 cup stuffing into bottom and up sides of each of 12 muffin cups heavily coated with cooking spray. Pour egg evenly into stuffing cups. Layer cheese, sausage, bell pepper, if desired, and green onions evenly over egg.
5. Bake at 400° for 18 to 20 minutes or until centers are set. Let stand 5 minutes before serving. Run a thin sharp knife around edges of muffin cups to loosen. Remove from pans. Serve immediately.