Add broth, marinara sauce, oregano, and cinnamon. Stir to combine, cover, reduce heat to medium-low, and cook for 15 minutes. Uncover and, if the sauce has thickened a little more than desired, add 1/4 cup of additional broth, cover, and cook for another 3–5 minutes. Otherwise, adjust salt and pepper to taste, and serve plain or garnished with crumbled Feta cheese.