Heat the oil and butter in a large saucepan over medium heat. Add the cabbage, scallions, garlic, and salt and cook until the cabbage is slightly softened, about 5 minutes. Remove 1/3 cup of the cabbage and set aside for garnishing. Add the chicken broth and potatoes and bring to a boil. Simmer, covered, until the cabbage is tender, about 15 minutes. Using a blender, puree until smooth and season to taste with additional salt, if desired. Before serving, garnish the soup with the reserved cabbage.