In an 8-quart soup pot, heat oil over medium heat. Add onions, carrots and potatoes. Cook, stirring occasionally until tender not not browned, 10 to 15 minutes. (Add 1 to 2 tablespoons water if needed to prevent scorching.) Add sherry and cook until liquid evaporates, 3 to 5 minutes. Add chicken broth, pumpkin (or squash), cinnamon stick, bay leaf and thyme. Bring to a boil, reduce heat and simmer, uncovered, until vegetables are tender, 35 to 45 minutes. Remove bay leaf and cinnamon stick.
In a blender or food processor, puree vegeable mixture in 4 batches. Return to pot and warm over low heat. Add nutmeg and season with salt and pepper. (The soup will keep, covered, in the refrigerator for up to 2 days. Reheat before serving.)
Ladle soup into warmed bowls; garnish with a swirl of yogurt and sprinkling of pistachios.