Nan's Cream of Celery Soup

2 mugs finely diced celery
1 slice of small onion
2½ cups milk, heated to hot (evaporated milk works best)
1½ cup fresh cream (room temperature)
3 Tbsp butter
3 Tbsp flour
1½ tsp salt
½ tsp pepper

Cook celery and onion with 1 cup milk in the top of a double boiler until tender, about 20 minutes. Remove onion.
Melt butter, blend in flour, salt and pepper, add remaining milk gradually, stirring constantly. Heat to boiling and cook 3 minutes.
Add celery and milk mixture.
Serve hot, garnished with chopped parsley.