PREPARE VEGETABLES: Wash and stem the spinach. Peel and chop onion 1� cups. Peel and chop potato. Chop dill, if it is fresh. Heat oil. Saute the onions on low heat for about 10 minutes. Add potatoes, salt, dash of pepper, and stock (or water). Bring to a boil, reduce heat, cover, and simmer about 10 to 15 minutes, until potatoes are tender. Add the spinach. When the spinach is wilted but still bright green (2 to 3 minutes), remove the pan from the heat. Stir in the milk and dill. Blend in a food processor or blender, in batches, until the mixture is very smooth. Whisk in the yogurt. Chill about four hours. The soup will thicken as it chills. If it is too thick, whisk in some milk, stock, or water. Taste and adjust the seasoning, if necessary. Serve garnished with a grating of nutmeg.