To make the sauce, heat the oil in a wide heavy saucepan over medium-high heat. Add the onions and cook, without browning, until softened, about 5 minutes. Add the garlic, tomato paste, cinnamon stick, and saffron and stir well to incorporate. Season generously with salt and pepper and allow to sizzle for 1 minute. Add the broth, bring to a simmer, and simmer gently for 10 minutes. Remove from the heat. The sauce can be made up to a day in advance and refrigerated.
