Fold top edge of a Medium Ziploc® brand Zip 'n Steam® bag out and down to keep bag open. Crack 3 eggs directly into bag. (Alternately, crack eggs into small bowl and pour into bag.)
Add the mushrooms, cheese, tomatoes, butter and salt to the bag
Seal bag. Squeeze eggs to break yolks. Continue squeezing or gently shaking bag until ingredients are thoroughly combined.
Place bag in microwave. Cook on full power for 2 1/2 minutes or until eggs are fully cooked and firm throughout with no liquid egg remaining. If needed, continue microwaving at 30-second intervals until omelet is done.
Allow bag to stand for 1 minute before opening.
Carefully open bag and slide out omelet. Fold omelet and garnish with chopped parsley if desired.