1Saute garlic and onion in melted butter in a large saucepan until tender. Stir in potato, salt, pepper, and 3/4 cup water. Bring to a boil; cover, reduce heat, and simmer 15 minutes or until the potato is tender.
2Finely chop watercress stems; coarsely chop the leaves.
3Add the stems and half the leaves, milk, and 1 1/2 cup water to the potato mixture; cover and simmer 15 minutes.
4Spoon the mixture into the container of an electric blender; process until smooth. Return the mixture to the saucepan; cook until thoroughly heated.
5Combine the egg yolk and half and half; gradually add to the soup mixture, stirring constantly. Cook, stirring constantly, until slightly thickened. Sprinkle with the remaining half of the chopped watercress leaves.