Melt margarine in a soup pot over low heat. Add onion, carrot and garlic. Cook, covered for 20 minutes, stirring occasionally, until vegetables are tender and wilted.
Crush tomatoes in their liquid and add to the pot along with the chicken broth, potato, basil, nutmeg and salt. Cook, covered, over low heat for 40 minutes, stirring occasionally.
Let cool to room temperature. Puree in a blender, in small batches, adding a bit of milk to each batch. Return to soup pot and adjust seasonings. Before serving, warm through over very low heat. Do not boil.