Stir in parsley, red pepper flakes, salt, pepper, and cinnamon, cover the pan and cook for 15 minutes, until the sauce is thick and flavorful.
Remove the cinnamon stick and push the sauce through a fine-mesh sieve or vegetable mill or process to a coarse puree. Return the sauce to the rinsed-out pan over medium heat. Cut the monkfish into 6 serving-sized pieces and add to the simmering sauce. Cook, turning the fish pieces until the fish is cooked through but still firm, about 7 to 10 minutes.
Remove from heat and serve immediately, spooning sauce over fish pieces.