HEAT OIL IN PAN over low heat. Add onions and cook, stirring, about 12 minutes. Add beets and cook, covered, about 35 minutes, stirring occasionally, until beets are falling apart. Add stock and salt. Cover and bring to boil. Reduce heat and simmer 15 minutes. Strain and reserve liquid. Transfer beets to blender or food processor and puree until smooth. Return puree to pan and add reserved liquid. Pour hot soup into tureen and ladle soup into bowls in front of guests. Decorate each soup bowl with 2 tablespoons yogurt and sprinkle yogurt with chopped dill.