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.