In a saucepan cook onion in oil over moderate heat, stirring, until softened, about 10 minutes. Stir in romaine and cook, stirring, until wilted. Stir in 1 1/2 cups broth, peas, sugar, marjoram, nutmeg, and salt and pepper to taste and simmer soup 10 minutes.
In a blender purée soup in batches until smooth, adding additional broth to thin to desired consistency. If serving soup hot, return it to pan whisk in sour cream. Heat soup, stirring, until just heated through, but do not let it boil. If serving soup cold, transfer it to a bowl and whisk in sour cream. Set bowl in a larger bowl of ice and cold water and chill soup, stirring occasionally, 15 minutes, or until cool.
