Mix madzoon, egg and salt in a two quart saucepan. Beat for 3 minutes. Cook on fast fire, stirring until it boils. Add the noodles, lower the fire and cook until tender. Saute the onions in the butter in a frying pan until light brown, then add the mint. Pour into soup, cook together for 5 minutes. In case soup becomes too thick for next day's serving add some boiling water. Note: Egg mixed with madzoon prevents curdling.