In a large heavy pot, heat the oil over medium-high heat, swirling to coat the bottom. Cook the onion for 6 to 8 minutes, or until very soft, stirring occasionally.
Stir in the ginger, garlic, thyme, cinnamon, pepper, and salt. Cook for 1 minute, stirring constantly.
Stir in the flour. Pour in the broth and water. Using a spatula, scrape the bottom of the pot to dislodge any browned bits.
Stir in the pumpkin. Bring the mixture to a boil. Reduce the heat to low and simmer for 10 minutes.
Stir in the milk. Remove from the heat. Ladle the soup into bowls.
Garnish with the sour cream and chives.
