Peel and chop sweet potatoes into approximately 1-inch cubes. Dice onions and celery. Mince garlic.
Heat butter or oil in a dutch oven or large pot over medium heat. Once sizzling, add diced onion and celery and saute for 4-5 minutes or until translucent. Add garlic, sage, ginger, and salt and pepper to taste. Saute for 1 minute longer until the garlic is fragrant, stirring frequently.
Pour vegetable broth into the pot. Add sweet potatoes. Bring to a simmer and cover. Simmer, covered, over medium-low heat for 10 minutes or until the sweet potatoes are easily pierced with a fork. Add pumpkin puree. Remove from heat and blend in the pot with an immersion blender (or with a high-speed blender) until smooth and creamy.
Sprinkle with additional dried sage if desired and stir in sour cream when serving.