In a saucepan over medium-low heat, gently sweat the garlic in the olive oil until it is very soft but not browned. Add the onion and continue cooking 5 to 7 minutes, until the onion is soft and browning. Increase the heat to medium-high and add tomato halves, cut sides down. Cook the tomatoes, turning frequently, until they have released their juices and are very soft.
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.