Heat oil in large soup pot. Add onions, carrots and celery; cook until softened, about 5-6 minutes. Add garlic, chili powder, cumin, oregano, and coriander. Cook until fragrant, about 1 minute. Add black beans, kidney beans, sweet potatoes, tomatoes, harissa, and stock; bring to a boil, then lower heat to medium-low and cook, covered, for 15-20 minutes until the sweet potatoes are tender. Add farro and cook another 10 minutes. Season to taste with salt and pepper. Top with chopped scallions and serve.