- Place oven rack in center of oven; preheat oven to 375° F.
- Remove portabella stems; wipe clean with damp paper towel.
- Spray baking sheet with cooking spray, and place mushroom caps on baking sheet.
- In a mixing bowl, whisk together all remaining ingredients, except 1 tbsp Parmesan cheese.
- Coat 10-inch non-stick pan with cooking spray and heat over medium flame.
- Cook and scramble egg mixture until it just starts to thicken. Remove from heat.
- Using a large spoon, scoop partially-cooked, hot egg mixture into portabella caps.
- Sprinkle tops with remaining Parmesan cheese.
- Bake about 20 minutes. Serve immediately.