Combine the water and cornstarch in a bowl and whisk it until there are no more lumps. whisk in the tomato paste, balsamic, mustard, brown sugar, hot sauce, and cinnamon.
Warm the oil in a large skillet (cast iron is best) over medium high heat. Add the onions and sprinkle with the salt to pull the moisture out. Move them around occasionally with a wooden spoon so they don't burn. Cook until the edges start to brown.
Add the garlic and cook for another minute.
Add the liquid, stir, and rub the pan with the wooden spoon to scrape up all the flavorful fond, the brown bits on the bottom. Turn the stove to low and simmer with the lid on for 1 hour. Check frequently to make sure it is not burning and the water has not evaporated. Add water if needed. The final result should be thick, not runny, but not pasty. After an hour, taste and adjust salt and other flavors as you wish.