To the oil remaining in the pot, add the onion, garlic, cinnamon, finely chopped parsley leaves and stems, 1½ teaspoons salt and 1 teaspoon pepper. Cook over medium, stirring often, until the onion has softened, about 6 minutes. Stir in the tomatoes with juices and 1½ cups water, then bring to a boil over medium-high. Stir in the couscous and shrimp. Return to a boil, then cover, remove from the heat and let stand until the couscous is tender and the shrimp are opaque throughout, about 10 minutes. Stir to combine and fluff the couscous, then taste and season with salt and pepper.