Heat the oil in a large heavy casserole. Add the onion, bacon and minced garlic; cook over moderate heat, stirring occasionally, until the onion is softened, 5 minutes. Add the potatoes and cook, stirring occasionally, until crisp-tender, about 8 minutes. Add the reserved chickpeas, stock, reserved cooking liquid, crushed red pepper, saffron and black pepper and bring to a boil. Simmer over moderate heat until the potatoes are tender, 10 to 15 minutes. Add the spinach and cook for 5 minutes. Season with salt and pepper. Ladle the soup into bowls and serve.