Saute the mushrooms in the butter until tender.
Remove from the heat and stir in the egg and oregano.
Stir in the cheddar cheese.
Spread about 3 Tbls of the mixture on each slice of toast.
Top each slice with 2 slices of tomato and sprinkle with celery salt to taste.
Broil for 5 minutes, or until the cheese melts and the sandwich is thoroughly heated.
Garnish with the parsley and serve.