HEAT OIL IN A POT over medium heat; add onions. Cook, stirring, for 5 minutes. Add stock, potatoes, nutmeg, salt and pepper. Raise heat to high, cover and bring to a boil. Reduce heat to low and simmer, uncovered, for 30 minutes. Remove from heat and puree the soup in batches. Replace pureed soup in a pot and reheat, covered, over low heat. To serve, place a dollop of cranberry sauce and sour cream in the bottom of each soup bowl. Transfer piping hot soup to a tureen or pitcher and ladle into bowls at the table.