Heat oil in a Dutch oven over medium-high; add scallions, garlic, and ginger. Cook, stirring often, until soft, about 3 minutes. Add broth, soy sauce, and gochujang; bring to a boil. Reduce heat to low, and add tofu and kimchi, stirring once, until heated through, about 3 minutes. Sprinkle with Korean chives, if desired.