Stir in parsley, red pepper flakes, salt and pepper, to taste, 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. Add the sauce to a clean pan and place over medium heat. Cut the monkfish into 6 serving-sized pieces and add to the simmering sauce. Cook, turning the fish pieces until cooked through but still firm, about 7 to 10 minutes.
Remove the fish to a serving platter cover with the sauce. Serve immediately.