Heat oil and butter in a large saucepan over medium heat.
Add cabbage, green onion, garlic, and ½ teaspoon salt. Cook until the cabbage is slightly softened, about 5 minutes.
Add the broth, potatoes, parsley, thyme and 1/2 t salt. Simmer, covered, until the potatoes and cabbage are tender, about 15-20 minutes.
Let cool approximately 15 minutes. Using an immersion blender, puree the soup until smooth. Once smooth, stir in sour cream. Add additional salt if needed. Serve immediately. Top with a dab of extra sour cream and a sprinkle freshly ground pepper.