Green Pea Soup

2 cups shelled peas
4 cups water
1 small onion, minced
2 small potatoes, peeled and sliced thin
1 tablespoon butter or margarine
1 egg yolk
Salt
1/2 teaspoon vetsin

Boil the peas until they are tender. Drain and save the water,
about 4 cups. Press the peas through a fine sieve, discarding the skins.
Brown the onion in the butter. In a saucepan put the cooking water,peas,onion, butter and sliced potatoes. Season with salt and vetsin. Bring to a boil and simmer until the potatoes are dissolved or mashed. Stir in the well beaten egg yolk.