Step 1 Peel the tomatoes: With a sharp paring knife, cut a small X at the base of each tomato. Bring a large pot of water to a boil, then drop the tomatoes in the water. Remove the tomatoes with a slotted spoon when the skin has just begin peeling from the X, about 20 to 30 seconds. Immediately place the tomatoes in an ice bath to stop the cooking, then peel the skin from the tomatoes when they are cooled.
Step 2 Quarter the tomatoes and remove the seeds, then roughly chop the tomato flesh and place in a large bowl.
Step 3Slice and seed the plums. Toss the plums and brown sugar with the tomatoes, then set aside.
Step 4In a wide-bottom pot, heat the olive oil over medium heat until hot. Stir in the onion and cook until tender, about 5 minutes.
Step 5Stir in the garlic, paprika and Marash pepper and cook until aromatic, about 2 to 3 minutes, stirring constantly.
Step 6Stir the plum, tomato and brown sugar mixture into the pot and increase the heat to high, stirring frequently. Continue cooking at high heat; the ketchup will bubble fast and furiously. When the major bubbles subside and the ketchup starts to thicken, reduce the heat to medium.
Step 7Stir in the salt, pepper, molasses and cinnamon and continue stirring. Taste the ketchup and adjust the seasonings and flavorings if desired, then remove from heat and cool slightly.
Step 8Using a food mill or immersion blender, puree the ketchup until all the chunks are gone and you have a smooth paste.
Step 9Refrigerate the ketchup or can according to the jar manufacturer's instructions.